Junk Miles - 5th August 2020

Simon - running coach
by Simon - Middle-distance runner and athletics coach

What are junk miles?

I went for a very slow run today. I'm not injured or ill or even tired. I just felt like running slowly. This got me thinking about "junk miles", and what a junk mile actually is.

If pressed for a definition at the start of my run I would have said something along the lines of, "Junk miles are miles run so slowly that they don't contribute to fitness improvements".

But how slow is too slow? Was today's pace slow enough to count as junk miles? I don't think so. In fact, I suspect that any aerobic exercise, regardless of how light it was, would have been better for me than doing nothing today.

So when would slow running be detrimental to my fitness? Obviously, if I were carrying an injury that was made worse by running, if I were sick, or if I were recovering from a previous running session, then slow miles may have a negative impact. But that would be true for running at any pace. In fact, I've had injuries, illnesses and recovery periods where very gentle activity - including slow running - has done me more good than faster running or complete rest. On those days I could define junk miles as, "miles run so quickly that they don't contribute to fitness improvements". But that doesn't really seem in the spirit of what most people think of when they think of junk miles.

However, assuming I'm in good health and well-recovered, then it's usually true that running at very slow speeds won't be as beneficial as running at higher aerobic speeds. E.g. a nice easy run at 2 mins/mile slower than 10k pace compared to a painfully slow run at 4 mins/mile slower than 10k pace.

So, sometimes slow runs are better and sometimes faster runs are better.

Maybe the definition of junk miles should be, "Miles run at a significantly-less-than-optimal pace".

That definition would cover both paces that are too fast and too slow. But that also doesn't really seem in the spirit of what is meant by junk miles.

Is there a pace that is so slow that it won't bring any benefits? In the context of a person who runs regularly and for whom recovery forms an essential and regular part of their overall training plan, then I think there is.

Very slow running will:

  1. Bring very minor fitness improvements
  2. Take the place of a rest opportunity
  3. Require some recovery time

For regular runners it's likely that when running very slowly the negatives outweigh the positives. Of course, faster running is also costly in terms of recovery required and missed opportunities to rest, but the positive training effect of this type of running shifts the balance in favour of getting out and eating up a few miles.

Have we answered the question? How slow is very slow? That's going to vary and can only be answered once you've considered things such as an individual's personal respsonse to different types of training, their fitness history, their recent training, what sessions they have coming up, how tired they feel, which injuries or niggles they're suffering, and so on.

Maybe there's not an easy answer or simple definition of junk miles. I would say that if you think you're running too slowly then speed up a little! And I don't mean more slowly than you'd like to run (we'd all like to be able to run faster), I mean if you feel like it's not really giving you benefits either in terms of fitness gains or recovery.

Take it easy (but not too easy),