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"Junk miles" is one of the most misused terms in running. A lot of people think it means their slow, easy runs, so they cut them. That is exactly backwards.
In this article, we'll break down what actually counts as junk and how to tell a useful run from a wasted one.
What a junk mile is
A junk mile is a mile run at a moderate effort: too fast to count as easy, and too slow to make you faster. Coaches call this range the gray zone.
As a result, it costs you recovery while giving little fitness back, so you finish a little worn down, with no real progress.
Remember: junk miles are not the slow, easy ones, but the medium ones.
Why easy miles are not junk
This is the part most runners get wrong. Easy miles are not junk at all, on the contrary, they are the foundation of distance running.
Junk miles began as a term from elite training, where athletes fine-tune every session and a wasted mile matters. It was never advice for everyday runners to cut their easy running.
Done at a genuinely easy effort, those miles build your aerobic base: more small blood vessels feeding the muscles, better use of fat for fuel, more mitochondria, a stronger aerobic system for the full distance.
Cutting those miles is a mistake.
Why junk miles slow you down
Junk miles kept up as a habit actively stall your progress for two reasons:
1) First, the effort is too high to fully build your aerobic base, the thing that lets you hold a faster pace for longer. Stuck in between, you train hard enough to stay tired but not enough to improve, so week after week your progress flattens. Plus, the constant moderate stress raises your injury risk.
2) Second, junk miles use up the recovery your hard days depend on. Show up to a hard session on tired legs and it comes out weaker than it should, so the quality runs that actually build speed do less for you.
This is why elite runners keep about 80 percent of their running easy and 20 percent hard.
How to tell if a run is junk
Telling a junk mile from a useful one comes down to one question: what is this run for?
Every run should have a job:
- An easy run is there to build your aerobic base, so it needs to be truly easy. That is heart-rate zone 2 (you can speak in full sentences).
- A hard run is there to build fitness and speed, so it needs to be genuinely hard. That is usually heart-rate zone 4 or 5, the range for dedicated speed work: intervals, threshold runs, hill repeats, fartlek, etc.
- A recovery run is there to loosen the legs the day after hard work, so it is very easy and short. That is heart-rate zone 1.
- A long run is there to build endurance and time on your feet, so the effort should be easy. It starts in zone 2, though your heart rate will drift up over the miles, which is normal.
- A marathon-pace run is there to practice race effort, so it holds a steady, moderate pace. That is heart-rate zone 3, which earns its place only because it has a specific job.
By contrast, a run at that same moderate effort (zone 3) with no clear job is the one to question.
What to do about junk miles
To run without junk miles, you should be a bit more purposeful and choose the right effort for each session. Here are a few tips to help:
- Slow your easy runs down. Keep them easy enough to talk in full sentences.
- Make hard days genuinely hard, and keep them few. One or two a week is enough for most runners.
- Give every run a job. Recovery, aerobic base, building speed. If a run has none, change its effort.
- Cut the moderate, no-purpose miles, not the easy ones. That is where the real junk is.
