A couple of things that bother me about this:
First, as Rita mentioned, how do the results with these runners compare to family members? How about the general population at similar ages? This “study” means nothing without a control group. It's incomplete at best, I'd prefer to use the word misleading.
Second, who says marathoning is a good idea for your health? Running or other aerobic exercise is beneficial to your health but a marathon is an extreme sport. I would fully expect it to be less beneficial to my health than a more moderate, non-competitive, running program. Of course, health is only one of many reasons I run so I personally don't care that fulfilling other reasons may not be as beneficial for my health as other paths I could take. I'm guessing I'm not the only one here who feels that way.