"But half of the older lifelong athletes showed some heart muscle scarring. The affected men were, in each case, those who’d trained the longest and hardest. Spending more years exercising strenuously or completing more marathon or ultramarathon races was, in this study, associated with a greater likelihood of heart damage."

http://well.blogs.nytimes.com/2011/03/09/when-exercise-is-too-much-of-a-good-thing/