Junk miles - is there such a thing?

Seems like the definitions and peoples understanding of "junk miles" in this thread varies a lot.

From any miles over a certain limit, to easy miles etc.

I would characterize junk miles as miles fast enough to impact your following sessions negatively, however not so fast that they are a substitute. Since these miles are detracting from your sessions you end up with less quality, with less recovery (and higher risk of injuries) and thus are a net negative in a training program.

I would think that any coach would not want to see that an athlete is never able to perform at key sessions, having sacrificed them to run at an intensity that is neither here or there.