It is my knees which will fail, first, I believe. I ran a good bit in my youth and my knees are not what they used to be. I have an uncle who had both hips surgically replaced. He basically worked himself to death on the oil drilling rigs. One of his hips had a hole completely through the socket joint in his pelvis and the cartilage was broken and lodged on the outside of his hipjoint. That was about ten years ago and he's still going at seventy something without hip problems.