The sorriest excuse for a training mythology yet foisted upon the endurance world may be the notion of "junk miles."
Before I explain to you why this is an utterly nonsensical theme, let me state the one, narrow justification for this idea: In a single sport activity—particularly footracing—effort levels that keep a runner from producing subsequent quality workouts (when quality is asked for) are not helpful. Running easy runs too hard might turn important runs into "junk," rather than allowing those important runs to serve their crucial purposes.
But even this is overstated, because, one man's junk is another man's treasure. In point of fact, these "junk" efforts occur every day at high-profile NCAA Division I running programs. Without singling out any particular college, a lot of 4:08 high school milers are burnt to cinders because of daily efforts executed above effort levels that are prudent.
Why does this happen? Because runners who used to rule their high school fiefdoms are thrown into the cauldron with runners at or above their levels. Instead of training at a pace and effort calibrated to each individual (which is what happened in high school), they train at the pace of the enclave.
Nevertheless, these enclaves of 15, 20 or 25 top high school runners may produce 6 or 8 superior runners, and those hard efforts aren't junk to the "overcomers": they're gold.
But I think the term "junk miles" has conjured an image among triathletes, that any "miles without portfolio" are to be avoided—that if you can't argue a thesis defining the virtue of a given workout, then it's junk.
In fact, what triathletes badly need is more junk miles. They suffer from not enough junk miles. Their performances suffer for the lack of junk miles.
This, because triathletes tend to have too many workouts that do have a raison d'être. If you think of the quality (high intensity) workouts that you do in the pool, on the bike, and in the run—the masters workouts, the group hammer sessions, the time trials, intervals, what have you—there seems precious little time for just the easy run.
Yet, if you total up your mileage, and especially your run mileage, you'll find that the sum looks pretty paltry if you only engage in "important" workouts.
When I'm asked by those entered in a Ironman what their training ought to look like, I immediately focus on the imperative workouts. These are the workouts that need to take place. Chief among those are the long bike rides, and, I'm talking about the 4 to 6 rides of 5 to 7 hours (each of which ought to be between 90 and 125 miles) that should transpire anywhere from 2 weeks to 12 weeks out from the race.
Then there are the long runs, the much shorter interval sessions on the bike, the two or three high-mileage swim weeks, and, presto, there's what's crucial in your campaign leading up to the race. But these might only comprise a third of all the workouts you'll do during that campaign. What about the other two-thirds? Junk miles. This, because you need a bridge of activity to get you from one workout "with portfolio" to the next.
For this reason, most successful pure runners do not rely on one one run per day. But they only rely on one hard run per day, and two hard runs out of every three days is more like it. Still, there's that morning or afternoon run of 5 to 10 miles. Were these runners only running their purposeful workouts and nothing more, they would probably be 40 or 50 mile-per-week runners. But in fact it's almost impossible to be national or world class without running 70 miles-per-week at least, and 90 or 100 is more like it, with occasional weeks of 130 and more.
Triathletes do not suffer from lack of intensity. They have plenty of purposeful workouts. It's just raw miles they lack, especially in the run. The miles needed to generate technique, leg toughness, to keep to a good weight, and to generate fitness—the "junk miles" that raise your weekly averages from 10 or 12 miles to 20 or 30 miles—are what is lacking.
Miles that cause overtraining are bad miles. Likewise miles engaged in when an athlete is sick or injured, or when the plan calls for rest, or for an off day.
But I don't think this is what is contemplated when i hear a derisive reference to "junk miles."
Indeed, junk miles are more than just allowable. They're probably the differentiator between acceptable, versus superior, race ability. The culprit is never junk miles, rather it is the failure to keep to the plan; the failure to keep intensity levels and sessions down to what's called for in the plan. Junk miles are the glue that bind each of your purposeful workouts to the others.