Cycling is a lot more forgiving than the other two endurance sports, swimming and running. The risk of injury or overtraining is less. The reason is that a cyclist has a lot more control over his or her level of exertion than a swimmer or runner. On my bike, if I go a long at 15 mph, I am exerting myself slightly more than I would walking. And it's possible to ride even slower than that. But to swim or run at any pace requires a level of stamina just to keep swimming or running.