You've probably heard tragic stories about people who have literally dropped dead while running a marathon. The man who collapsed in the heat of the 2007 Chicago Marathon already had heart disease. The Harvard University Gazette published an article stating that marathon running can damage a heart - the problems primarily among inappropriately trained runners.
I'm a marathoner, and I've been training with people, including my coach, who have heart issues. We've all been made that much more aware of how cautious we need to be, as well as how strong we are making our hearts through properly guided training.
RunnersWorld online has a great quiz for runners who want to learn how much at risk they might be for developing heart disease. Are you a runner? Take the quiz:
http://www.runnersworld.com/article/0,7120,s6-241-285--11277-0,00.html