I'd say it depends on the circumstances. Some of you guys seem to take for granted that anyone can just build up to running 50+ mpw and it just comes down to willpower and desire.
I take running pretty seriously for someone with a full-time serious job, but I've been dealing with a lot of injuries over the last year and my body seems breaks down when I try to increase my mileage
I don't know what is or is not serious but I absolutely DO believe that anyone who can run can build up to 50+ MPW if they do so intelligently: i.e. slow-ish progression, not trying to build in too many workouts while building up, understanding that recovery days are to be run at a proper EASY pace. 50 MPW is really not much, especially if you do a reasonably long run on the weekend. If you're always getting injured during buildups, IMO it's almost certainly because you're trying to rush things in some way.