Below is a great (quick) article about recovery and stress. It emphasizes something I have been following very closely the past 2+ years: stress. Training for a triathlon, running race, or other physical endeavor elicits a stress to the body. The goal of training is to stress the body then allow for a period of recovery to allow for adaptation. This adaptation allows the body to become stronger, faster, and have better endurance.
If you have too much stress and not enough recovery,