Junk miles are usually defined as ancillary mileage with an unacceptable risk-reward trade off. It usually deals with running “excessive” mileage, but it’s not the only case, in my opinion. Many athletes spend their time running in no mans land where it is too fast to be aerobic work but too slow for a tempo. For sprinters it’s a run that is too fast for aerobic work, but too slow for quality speed work. That's not to say that running in between energy systems will not make you faster, but the costs outweigh the benefits because you incur a lot recovery time for very little benefit. You’d be better off slowing down or speeding up and getting in a real workout. . . The appeal about training in no mans land is that it feels good. Track is life and like most things in our lives that feel good are probably wrong. There you have it... it only takes one step with the wrong intention in order for it to equate to junk. . .