A recent post by Scientific American reports on a study concluding that endurance events, including marathons, ultra marathons, and long-distance cycling, might cause scarring the heart, leading in some cases to a fatal cardiac event. See http://blogs.scientificamerican.com/observations/2012/06/04/ultra-marathons-might-be-ultra-bad-for-your-heart.
While fatalities in marathons and ultramarthons are rare events, a long-term study of 52,000 runners found that those who only ran one to 20 miles a week spaced out over two to five days and at an 8.5- to 10-minute mile lived longest. (Note: other lifestyle factor in this group were not reported, so running may or may not have been the operative factor).
The good news for followers of Smart Marathon Training, though, is that targeted truing still seems to be there nest way to go. “Indeed,” reported Scientific American, “the best bang for your buck seems to be a daily short session—just 20 minutes or so—of intense, interval-based exercise, such as repeated bursts of intense running, biking, swimming or strength training with short recovery periods in between.”