Seoul, Korea, 1988: Jacqueline Gareau's last good shot an an Olympic medal in the marathon. A former Boston Marathon winner, Gareau was in top shape and champing at the bit. Yet she didn't even make it to the starting line that day because of a bad cold- a cold that was the direct result of doing too much without adequate rest. Gareau had overtrained.
Stop, look and listen. Signs of overtraining can be subtle, but experienced runners who have learned to listen to their bodies are aware of the typical warnings.
An easy cure. Go to the movies. Read a novel. Bake cookies.
In other words, take a day off now and then to give your body a chance to recover. Marathoner Steve Spence, for example, takes a day off every two weeks, regardless of his training. You can't lose your conditioning in one day, and you might ensure yourself more enjoyment and better performance in the future.