I look at all this with a very cynical biased mind. I hate running and don't do it in any form for pleasure. I have never had this "runner's high" that is talked about in that Michael Moseley piece (the high that i get is the same as I get bashing my head against a brick wall - it's so nice when the pain stops). Given this I can't see that studying people who run marathons says anything about anyone other than people who have an ability to run marathons. (Given the thousands who go in for popular marathons, I would have thought that the 249 mentioned is a bit small to be a representative sample.) If the sport is meant to be "high intensity" surely studying 200m and 400m runners in an athletics tournament would be more appropriate (or rugby players and american footballers for example).
I note that in professional cricket fast bowlers have a remarkably high rate of breakdown in spite (or perhaps because) of the high intensity of their sport, and that with all the sports medical support that helps them.
If I listen to my GP then the bulk of the studies on arthritis conclude that a major factor is your parentage. (He runs - but he isn't selling it as an activity - just says the important thing is staying active, regardless of the pain, because the pain is never going to go away)