Most runners seeking to maximize their 5k or 10k potential will need to adopt some form of interval training. After a base of solid distance running has been established , a runner can add interval training to complete the elements needed for optimal racing fitness.

Purposes of Interval Training

Understanding the principles and purposes of interval training will guide runners in developing workouts that are tailored to their particular situations. By interval training, I am referring to workouts in which hard running efforts of prescribed distances or time are repeated with intervals of rest between the repeated hard efforts. The term "interval" actually refers to the rest interval, but will be used here, as elsewhere, to describe both the running and rest portions of the workout.

There are three main reasons that interval training is done:

1. Intervals are used to increase anaerobic threshold levels. By repeating sustained hard efforts at near anaerobic condition, the runner improves his ability to run hard without going into oxygen debt.
2. Interval training also increases a runner's endurance. This means that the runner can continue at a certain pace for an extended period of time.
3. Finally, interval training builds muscle strength. Typical distance running exercises the leg muscles in a certain range of motion, with the focus on slow-twitch fibers. By running at faster speeds, the runner exercises all leg muscles and improves flexibility during running, both of which will mean improved muscle performance in races. This makes running at a race pace easier and improves top speed for sprint finishes.

While these reasons can be summed up by the maxim, "if you want to race fast you have to train fast," they also indicate (at least reasons 2 and 3) that some small amount of intervals will also be a benefit to even fitness joggers.

The amount and distance of the intervals, as well as the frequency of the training sessions, will be determined by the quality of mileage training, the type of runner involved, and personal preference. Two principles must be kept in mind when developing a training schedule that includes intervals: (1) the intervals must complement the distance mileage training (i.e., a runner needs to identify what is missing from the mileage running), and, (2) the type of workouts must suit the runner both physiologically and psychologically.

The latter point is important, especially for runners who are no longer part of a team. It is hard enough for a runner to motivate himself to do a tough workout, let alone one that the runner does not like or do well at. In short, for a runner to benefit from interval training, he has to show up at the track. And to reliably and enthusiastically show up at the track, the runner must have interval workouts that work for him.

Like many runners, I sought the perfect workout, or combination of workouts, to achieve ultimate fitness. It took me a while to click my heels three times to find my way home on this. Within a defined need for fitness development, there is a great deal of latitude in the type of workout that can provide that fitness.

Physiological Differences

Different types of runners will benefit from different mixes of interval training. A runner with a greater amount of slow-twitch muscle fibers will generally do better with longer intervals. Conversely, a runner with a higher percentages of fast-twitch muscle fibers will tend to do better with an interval mix that includes more shorter intervals.

The slow-twitch runner will generally need fewer interval sessions than the fast-twitch runner. Indeed, too many interval sessions can quickly fatigue the slow-twitch runner's limited number of fast-twitch muscle fibers, which results in no staying power in longer races (the appearance of no endurance). Runners with more fast-twitch fibers will generally thrive on more interval sessions. For example, it is plausible that a slow-twitch runner would need no more than one well-designed interval session per week, whereas a fast-twitch runner would need three weekly interval sessions to maximize his ability.

That being said, slow-twitch or fast-twitch, a runner who is seeking to maximize fitness for a 5k or 10k will need to have a significant amount of high quality mileage or longer intervals. Most runners who have trained on a team have seen a "workout king"––a runner who excels at interval training, especially shorter intervals, but fails to come close to the same level during races. The usual cause of this dissonance is a lack of endurance, which can only come from quality mileage or longer intervals.

Another caution when it comes to interval training is that runners can compete with other runners or themselves during these sessions and lose sight of what they are trying to accomplish. Three workouts come to mind, one where I ran 4 x 1-mile and averaged 4:19, one where I did 20 x 400 meters and averaged 64 seconds, and one where I did 20 x 200 meters and averaged just under 30 seconds. These all sound impressive and they were performance goals in my mind, but they were three of the most worthless workouts I ever did because they were too strenuous and did not help me build toward the three purposes of interval training (anaerobic threshold, endurance, muscle development). For other runners, those workouts may have been appropriate, but for me they were counter-productive (note that I was ill soon after each one of them).

Complementing Distance Training

The runner who has plenty of quality mileage––runs pretty hard for much of his training, includes hills in his runs, does tempo or steady state runs––will have less need for intervals than the runner who runs easy mileage. For example, in the classic Lydiard training regime, a base of quality mileage is followed by hill repeats and then by a period of shorter intervals (200 meters to 400 meters). This works because the runner has the endurance and a high degree of aerobic fitness from his mileage. The shorter intervals add the final piece to the mix and further muscle development.

In contrast, under the old Oregon system, the endurance and anaerobic threshold were built on longer intervals (800 meters and up) and on shorter intervals with abbreviated rest periods or moderate speed recovery periods (similar to fartlek). The quality of the mileage was less important because so many other elements of fitness were obtained from the interval training.

The current world-class African runners tend to have a system more like the classic Lydiard program––plenty of mileage (100 miles per week and up), much of it at a fast pace, plenty of hills and fartlek. This mileage is combined with a limited number of long and short interval sessions throughout much of their training.

Longer Intervals

Longer intervals can be done in several ways. The most basic is to do a set of intervals of the same distance at a prescribed pace. For example, many runners like to do 4 x 1-mile at a set pace (usually at about 10k race pace). This brings the heart rate up to the desired level and holds it there for several minutes during each interval.

The longest interval should rarely exceed 2 miles. Beyond that point little is to be gained from the workout that a simple tempo run (steady run at prescribed pace) would not provide better. Also the recovery time to effectively run really long intervals must be substantial in order to have quality hard efforts, and so the rest period itself becomes a problem. I can recall doing 3 x 1.7 mile cross country loops where the rest period had to be 15 minutes to allow me to put in a good effort on the run. I would have been better served by simply doing a 5-mile tempo run.

The shortest "long" interval would be about 800 meters. It is safe to say that most runners seeking to maximize their 5k or 10k potential will need to regularly do intervals of 800 meters to 2 miles. Within that range, the runner is fairly free to choose the mix that works best for him.

To make these longer intervals more stimulating, varied pacing can be used. For example, do 400 meter laps of 80, 75, 70, 80, 75, 70 for a 6-lap interval at 5-minute mile pace. Two or three of these would be sufficient for a good workout for a 30-minute 10k runner. A variation of this that we had at Oregon was two laps at 80, two at 75 and then two at 70 (we actually ran faster than the schedule), and we did two or three of these. These speeds can be adjusted to reflect various performance levels (e.g., a 40 minute 10k runner might do laps of 105, 95, 85, 105, 95, 85 for an average pace of 6:40 per mile).

Some runners like to vary distances. Ladders are popular, for example doing distances of 200m, 400m, 800m, 1200m, 1600m, 1200m, 800m, 400m, 200m. Another strategy is to mix a longer interval with a shorter interval. Taking another example from Oregon of one of my favorite interval workouts, we would do an 800m hard effort, followed by a 300m jog, followed by a 300m hard effort, followed by a 200m jog, which we would repeat three or four times. These would be done in a "cutdown" manner described below.

For runners who get tired of doing intervals on the track there are options. Trail or grass loops can be used. Many bike paths have mile markings that can be used. Alternatively, the interval distances can be done by time on relatively flat terrain, with the hard efforts lasting 3 to 5 minutes.

Shorter Intervals

Shorter intervals are added to the mix to provide the final element of anaerobic threshold and muscle development. In sufficient quantities and at the right pace, shorter intervals can also provide some endurance building.

The interval distances can be as short as necessary to meet the athlete's needs. Lydiard would have his athletes do repeated 50-meter bursts to add the final sharpening for racing. It can be enjoyable and beneficial to do 100 meter repeats on a football field, or as "jog the turns, sprint the straights," on the track. However, these very short intervals should only be a small part of a 5k or 10k runner's regime.

The majority of short intervals will be of distances of 200m, 300m and 400m. They can be varied, like the longer intervals, or there can be a set amount at a specified pace. Like longer intervals, some runners prefer "cutdowns"––gradually reducing the time of the hard efforts (from relatively easy to very challenging).

Adding short intervals, such as 6 x 200m at a relaxed sprint, at the end of longer interval sessions or tempo runs can provide needed balance in developing overall race fitness.

Also as noted above, shorter intervals should not be the exclusive form of interval training, unless the runner does a substantial amount of quality mileage. An athlete who does easy mileage and only shorter intervals will likely never develop the endurance to maintain his potential over a 5k or 10k.

Amount of Rest

The interval of rest between hard efforts has historically been pegged at the time necessary for the heart rate to drop to about 120 beats per minute (bpm). This works fine for shorter intervals since it is typical that the rest period is about the same amount of time as the hard effort. However, for longer intervals, the heart rate will likely reduce to 120 bpm before the runner is physically ready to go again. Some runners simply jog one-half of the longer interval distance as a recovery period (and this takes about the same amount of time as the hard effort portion).

From this midpoint of recovery time, runners can vary the amount rest they take depending on their workout goal. Runners who are seeking to develop a greater amount of endurance can reduce their rest periods. Runners seeking to run faster during their hard efforts (usually to increase fast-twitch muscle development) can take longer rest periods.

Generally, within a reasonable margin of the midpoint recovery period (20-30% variation), it is simply a matter of preference of the runner to determine the amount of rest between hard efforts. The margin of fitness difference between a little more or a little less recovery period is inconsequential in the overall context of training.

The next question is whether to walk, jog or run during the recovery period. Again, this should be determined by the primary purpose of the training session. Walking obviously provides more rest than jogging if the rest periods are the same amount of time. Walking for recovery might make sense if the primary goal of the interval session is to maximize muscle development. If building endurance is the main goal, a walking recovery makes little sense.

Jogging during recovery is generally preferable to walking if for no other reason than the legs stay warm and loose between hard efforts, reducing the risk of injury during acceleration. Jogging during recovery also has the added benefit of keeping the runner running, which improves endurance and the mental toughness of the athlete.

Active running as "recovery" is used to build endurance. For example, a workout of 4 x 1-mile can have 1-mile recovery runs at 15-20% slower than the hard miles (e.g., if the hard mile is 6 minutes, the recovery mile would be 7 minutes). Prefontaine liked the workout which alternated 30 and 40 second 200 meters for 2.5 to 3 miles. This type of workout will improve anaerobic threshold, but like any fartlek, it primarily serves to increase endurance. And at certain speeds, this type of workout is very tough and not for everyone.

How Fast to Run Intervals

The speed at which a runner should do intervals is again influenced by the purpose of the workout and the athlete's other training. A runner who does easy mileage is going to need to get more out of his interval sessions and will likely run intervals faster than a similar athlete who does higher quality distance running. In addition, the speed is influenced by the volume of work to be done and must be consistent with the recovery time.

Going back to my example of doing 4 x 1-mile averaging 4:19, the reason the workout was not productive was because it was too close to my maximum ability. It simply broke me down too much, given the other training I was doing. Rather than being at race effort, the miles should have been done in the range of 80-85% of maximum heart rate, which would be about 10k race pace.

And pegging 1-mile intervals to 10k race pace is generally a good target for the typical runner. 800-meter intervals can be set at about 5k race pace. Intervals longer than 1-mile should be adjusted accordingly. If pace is being varied during the hard effort, the average pace is usually slowed. And for "cutdowns" the average pace is also slowed down slightly (e.g., the first mile repeat would be slower than 10k pace and the last mile repeat would be closer to 5k race pace).

The pace of shorter intervals varies much more widely. If a large quantity of intervals are to be done, the pace will naturally be slower than if fewer intervals are to be done. If building endurance and anaerobic threshold is the main focus of the workout, the intervals should be slower than if developing muscles is the main goal. For building endurance and anaerobic threshold, the speed of a 400-meter intervals should be slightly faster than 5k race pace. 300-meter and 200-meter intervals should be done at a slightly faster pace than 400-meter intervals.

With all interval workouts it is important not to go too fast too soon. It is far better to start slower and increase speed as the workout progresses than to do the opposite. To start the intervals at speeds that cannot be maintained reduces the quality of the workout and makes the recovery from the workout take longer. In addition, it is de-motivating to fail at a workout in this manner.

As race fitness improves, the speed of the intervals should keep pace, but should not exceed the race improvement level. Instead, it is better to add more hard work (e.g. add another mile repeat) rather than increasing the pace faster than the improving 10k and 5k average pace. This added hard work will add more fitness (better endurance and anaerobic threshold development) than doing less hard work at higher speeds.

Length of Total Interval Session

The total length of the hard effort in an interval session should be in the range of 2 to 5 miles. It will tend toward the lower amount with shorter intervals and toward the upper limit for longer intervals. Most runners find about 3 or 4 miles of hard effort to be the right amount of quality work in a single session. For example, typical workouts are: 4 x 1-mile; 12 to 16 x 400m; or 20 x 200m.

If the runner finds that the speed of his intervals is at the maximum recommended speed (see section above) and he is reaching the 5 miles of hard work, and he is not improving his times, one of two things are possible. First, if everything is being done right, he is in peak condition. Alternatively, the runner is turning into a workout king and needs to add more endurance training to be able to improve his race times.

The length of the workout will vary depending on other factors in the athlete's training. The greater the number of interval workouts, the lesser the amount of intervals needed in each workout. The higher the quality of the distance mileage, the lesser the amount of interval distance needed.

How Often to do Interval Sessions

Finally, the runner needs to determine how often to do interval sessions. This will depend on the time of season, how well the distance running has gone, and the physiology of the runner. For a slow-twitch athlete who is competing every other week and has a solid base of quality distance running, one long interval session and one short interval session during the fortnight should be sufficient, together with at least one other tempo run of 4 to 8 miles and several medium to hard distance runs.

At the other end of the spectrum, a fast-twitch runner who runs his distance mileage easy, will need far more interval work during the same time period. Such an athlete might need two long interval sessions, two short interval sessions, and two tempo runs during a two-week period.

Athletes who race more frequently need to reduce their interval training sessions to accommodate the increased races (which are also a form of quality training). If there are fewer races, an interval session can be added. As racing season approaches, it is better to slightly under-train than risk overtraining which causes fatigued muscles, requiring substantial rest for recovery.

Interval training provides a level of fitness that is difficult for most runners to achieve by doing distance mileage alone. As with much of training, flexibility is needed in designing and performing interval sessions. Every runner is different and will react individually to different workouts. But if the purposes and principles of interval training are kept in mind, the runner should both enjoy and benefit from interval training.