The two main things to watch for in overdoing it are injuries, like overuse injuries, or doing so much that your body can't recover in between workouts (overtraining).
For example: www.sparkpeople.com/resource/fitness_artic
Here is an interesting read by a cycling coach talking about training for *really* long rides: www.ultraracenews.com/2011/11/18/training-
I know that sounds pretty extreme, but the same general type of problems (injuries and overtraining) will happen to different people at different points. It is why people who know the most about problems always say to increase exercise slowly and be careful of injuries.
Edited by: BEEZAUR at: 5/18/2013 (02:32)