I think you'll find that cycling is easier on the body than running. I used to train for half-marathons, and running two or three hours in the Phoenix summers would wipe me out. However, I can ride three hours with no after-effects. It's less pounding on your body, plus you create a cooling breeze when you ride. Of course, it's all relative. If you really hammer on those pedals, you could overdo it. Just listen to your body, as you do when you run, and you'll know when to take a rest day. Cycling is a great way to cross-train--get yourself a bike that fits, and you'll fall in love for life.