I hear a lot about 100 mile weeks, but based on my experiences, it's physically impossible for many people to get to even half of that amount. I obviously don't have an exact figure, but my rough guess is that no more than 1 in 3 able bodied young men of a healthy weight can handle it.
It's simply a question of workload vs. recovery. Even if you slowly build up to it and take down weeks, there is a point where you simply cannot handle the miles without breaking down. Sometimes this takes the form of injuries, and sometimes this takes the form of a big performance drop. Either way, running 50+ mpw will not lead to good performances for most.
I know many people, including myself, who've failed to succeed on 50 mpw. Many attempts end in stress fractures, Achilles problems, and other injuries, but the ones that don't feel downright miserable. You know you need to eat more, but you have no appetite, and even junk food doesn't look appealing. You know you need to sleep and recover, but it's almost as if you're too exhausted and nervous to sleep.
It's like you're on a Himalayan climbing expedition or something. Every run feels like you're dragging a load of elephants even though you haven't gone anaerobic in months. Downhill runs feel like flat surfaces, flat runs feel like climbs, and shallow climbs feel like you need hiking poles to ascend them.
And then come the races or time trials. So far, you've avoided bringing your watch or running with anyone else because finding out how slow you are would be too demoralizing. But you can't avoid the clock forever. So you take one off day, one short easy day, and then step up to the line. You're way off from your PRs, get outkicked by people you know you should easily beat, and are left wondering the following:
Why should I even go through all that if it just makes me slower?