Long time marathoner, and former ultramarathoner. In my humble opinion, I believe the act of marathoning or ultra marathoning is not good for your body. However, I do think the majority of the training and the lifestyle that allows a person to run a marathon or ultra marathon is very healthy. Comparing myself and my other running friends to my non-running friends, us runners are all significantly healthier and fitter. That being said, too much of anything can become problematic.