Last week at the American College of Cardiology conference, an abstract was presented:
1197-358 - Are Cardiovascular Risk Factors Responsible for the U-Shaped Relationship between Running and Longevity? The MASTERS Athletic Study
The researchers found that those who run >20 miles/week (on average) actually see a decrease in their longevity. In other words, they die earlier. The underlying cause for this remains unclear and I'm sure that over time researchers will discover a scientific explanation, but for now, we should apply common sense: our bodies are not made to run 26 miles. So, maybe it might be OK to run a marathon occasionally, but if we make a habit of running frequent marathons, then perhaps we're damaging our hearts which leads to scarring, heart enlargement, and other abnormalities.