Swimming on the web
Does prolonged exercise damage the heart?
"Now, there’s no doubt that the benefits of exercise, like any drug or intervention, must ultimately follow a U-shaped curve; there’s a point where more is no longer better, and another point where more is actually worse. The challenge is figuring out where these two points are – and keeping in mind the difference between them."
To put things into perspective:
"In the key study, the average subject had been training seriously for 43 years, and completed 178 marathons, 65 ultramarathons and four Ironman triathlons."
Alex also addresses how much is enough to get most of the health benefit:
"In other words, if you already do 60 minutes of vigorous aerobic exercise every day, adding another 10 minutes isn’t going make you significantly healthier."
But that doesn't mean there aren't other benefits, including social and performance benefits.
See the article for more details.