I've run across several mentions of a new study in articles and on tv. Here's one article: http://www.kansascity.com/2012/06/03/3640410/extreme-exercise-may-carry-cardiac.html
I'm familiar with the concept of not bothering with exercise sessions beyond about an hour unless you are trying to improve at endurance sports that require more time than that, but I hadn't previously heard specific mention of heart problems possibly caused by regularly going well beyond that. I recall one article mentioning risks increasing when approaching mid-forties in age and when doing ultra distances/extreme endurance sessions more than once per week.