Honestly, why worry about this? Nobody ever died of Nothing before. I am simply going to run and ride because I enjoy it. I certainly enjoy it more than I would being over weight. I am sure that there is a happy medium as with anything but I am not going to worry. I would rather endulge myself in a little too much exercise than a little too much candy or couch time. At 47 I am dealing with my first real injury, a torn hip. My friends say it was my lifestyle as a kid and adult (too many sports and too much training). My Dr. is not sure is it is overuse or a bone defect that caused it. Either way, I will be back running again after my surgery. I love it and I love riding. I like to run/rode fast and at long distances. If that is my poison, than I am ok with that.