Here's a report on a study that monitored over 100,000 people:
Paul Williams, Ph.D., author of the study, found that men who ran two or more marathons per year were 41 percent less likely to suffer from high blood pressure, 32 percent less likely to have high cholesterol, and 87 percent less likely to be diabetic than non-marathoners. Those who ran only one marathon every two to five years also had significantly lower risk for these conditions than non-marathoners.
The benefits of running marathons were largely independent of total number of miles run per year by participants, indicating that isolated distance running bouts in preparation for marathons may have been effective in decreasing risk of disease. Even runners who didn't run marathons - those who included longer runs as part of their usual exercise routines - were less likely to have high blood pressure, diabetes, or high cholesterol.
http://www.medicalnewstoday.com/articles/140104.php
Here's a different piece on how regular hour-long runs stimulate neurogenesis and memory improvements in middle aged humans:
http://www.dana.org/news/brainwork/detail.aspx?id=7374
Distance running is one of the most beneficial things you can do for yourself. Racing marathons is also fine, with sufficient training.