This topic has been studied a lot. What is improtant for Marathoners is to keep a high training base (ideally >45miles per week), and do not push to a very high intensity (That means REALLY high!). Doing correctly, running is a good lifelong sport. The problem of those competing athletes is: They are not doing it as a lifelong sport, they are shooting for a short term goal with very high intensity training.
One of the good acticles is here:
http://runninginjuryfree.org/2008/09/effects-of-marathons-and-ultras-on-our.html
I am still researching more tips for better lifelong running,I appreciate if anyone sharing their thoughts.
Thanks.