It's interesting how people just assume distance running must be the cause of our problems. While I had run most of my life, I only fell in love with marathoning at age 42. I ran 8 marathons after that and absolutely loved it, and my times were actually improving into my late 40's. Unfortunately osteoarthritis (knee) and an unrelated knee surgery have put an end to that kind of mileage.
I can't count the number of times people have asked me “Don't you wish now you hadn't run so much?” Despite the fact that I know of no studies that show that running causes osteoarthritis, or that there is a higher incidence of it among runners, the assumption is that running caused it for me. My response is to point out that both of my parents have had both knees replaced as a result of the same disease, and my Dad had the same problems I have now, when he was in his 40's, just like me. If running caused mine, what caused theirs??? They weren't runners. And what about those men and women in their 80's who are still chugging along just fine and have put a lot more miles on their knees that I have???
I believe I was genetically predisposed and it would have happened anyway, running or no running. Regardless, there is no way I would give up the fun I had, the friends I made or the joy and sense of accomplishment I exerienced during my running life, even if it would make my knee feel better. Not a chance………