Here's an interesting article which gives insight into the possible dangers of excessive exercise,
http://www.telegraph.co.uk/science/6049198/excessive-exercise-could-be-as-addictive-as-heroin-claime-scientists.html
Marc
http://www.telegraph.co.uk/science/6049198/excessive-exercise-could-be-as-addictive-as-heroin-claime-scientists.html
Marc