I ran pretty seriously into my early 40's (60+ miles/week) training primarily for marathons. Could eat anything I wanted, never injured. I attribute the lack of injury to running 75% in the hills which works all the leg muscles and keeps things in balance. I'm early 60's and if I had some local buds who ran I would still like to run a few miles in the hills now and then, but you do get beat up - greater chance of injury, etc., etc. I also believe that in addition to the calories burned during exercise there is an overall elevation of the metabolism which would be the case with any cardio activity. I'm sticking primarily with cycling, but I'll jog or use an exercise machine when I travel.