I had patellar tendinitis in both knees. I was mid 40s and getting slower already. I thought it might be arthritis and just stopped running for about six months. Then, I started jogging around and my knees felt way better. But I flipped on my son's skateboard and got some weird strain on the outside of my left knee. Stopped running for another six months and then strained my right calf when a snowboarder turned in front of me on a mogul and I had wipe out to avoid hitting him. Another six months of no running to deal with that one. Finally, after several attempts to restart running just to keep from getting fat, I am out running 6 mi a day. Knees have never felt better. Might do a 5k if I can get in better shape and see how I measure up against the 50+ guys. But that is about it.