I second what tangent girl said. Sure, maybe running marathons or ultras "at your age" (like it's old ) isn't the best idea, but I train regularly with men (and a couple women) who started running in their late 40's/ early 50's and are still going strong- with no shot knees. One guy is 72 and still does IM races. And if you're only running 3-6 miles at the most, the training for that isn't going to kill your knees. My 62 year old running partner had a total hip replacement a year ago, and the doctor said that if he HADN'T been a runner he'd have had to had it replaced long ago. Second opinions are a good thing (especially from and orthopedic doc or a sports medicine doc).