Flea'd from DJ on the VS:
http://todayhealth.today.com/_news/2012/12/03/15625246-running-farther-faster-and-longer-can-kill-you?liteOf course this could probably be extended to any exercise which raises your heart rate considerably and is maintained for long periods of time without rest. Long distance swimming and cycling come to mind as well, though with cycling it's possible to rest without stopping so maybe it's less risky.
I doubt this will ever be a concern for me as I see a half marathon as the loftiest goal I ever could put in front of myself, but even at that point you're talking about almost 2 hours straight . . . long enough to be questionable according to the article.
