I have been seeing the term "junk miles” thrown around lately, clearly with the connotation of being bad because it's preceded by the word "junk". Reading into the context it's used, the writers are referring to slower, easy miles.
When did easy miles because suddenly something you must avoid? Aside, from Run Less, Run Faster, where did this shit come from?
Easy miles are the foundation and base of any distance (half/full marathon) training program. Stocking your training week with 3 to 4 fast/hard workouts only to follow them with some recovery miles and doing no easy miles is a great way to get injured. Sure, the elites might have 3+ hard workouts per week, including tempos, trackwork, fartleks, two-a-days, etc, but they have built up to that their whole lives. And they're elites! Genetically gifted runners!
Most training plans out there, do not advocate more than 1, maybe 2, tempo/speedwork days per week. The long run? Guess what… that is a hard workout. When you run 15+ miles, that is a hard run that requires a rest day or a recovery/easy run after. Long easy runs are not junk miles. Mid week easy run? Not junk miles. They are foundational/base mileage keeping you from getting injured during your hard workout days. Personally, when I do more than one speedwork day a week, I find myself riding a fine line between just barely healthy and injured. With a 50-50 chance of finding myself on either side.
Sure, I have not backed up these statements with any scientific studies or research links. They are anecdotal – my own empirical findings. However, I ran all 3 seasons of track in high school and, while I was always a middle-to-back-of-the-pack runner, our teams were consistently competing on the state and regional level (by regional, I mean all of New England). Our coaches weren't dumbasses. Our XC and middle/long distance track coach is now a head track coach at a university (and while not a huge school, it’s not a podunk super-small one, either). We ran easy miles ALL THE TIME. We had maybe two hard workouts a week. One on the track, one tempo or fartlek. If there was a meet that week, then maybe just one.
Calling easy miles "junk miles" is sending the wrong message, especially to newer runners out there. Wanna keep injuries at bay? Make easy miles the bulk of your training volume.