The first time I ran 60mpw I was tired all the time, few years later the first time I ran 90mpw I was tired all the time. My last two stints at 90mpw I didn't really get tired. I would guess that going forward you will not have issues with 80-100mpw because your body adjusts. So it will not be as much of a negative side effect of training.
Anyway, I have heard of a local(regional?) runner going sub 4 and immediately stepping away from running.