Hi Jeff,
You said in another thread viewtopic.php?f=22&t=43482 that you stopped running marathons when you learned about the evidence.
The Mayo Clinic paper you quoted is important insofar as it highlights that more-is-not-always-better. However, since the researchers start off with a long list of cardiac (and other) benefits that accrue to runners; since they say that benefits increase with dose up to 1 hour of exercise per day (which is about the amount of time spent by 90%+ of marathon runners); and since they aren't aware of an algorithm for determining how much is too much, practically the article won't change much behavior among runners. It seems that the few fanatics out there who are working out enormous amounts of time every day probably won't take notice since they are fanatics, and everyone else is within the known safe limits.
Isn't it safe to assume that marathon runners who train for about an hour a day and run marathons once or twice per year are within safe limits?