I ran the Boston Marathon in April 1971 as an unofficial runner (I think only 1,000 - 2,000 were qualified and numbered, and 10,000 other people ran just for the heck of it like me, but we never heard the term "bandit" runner 33 years ago). The proof that Kerry couldn't have possibly relaly run the Boston Marathon is that he referred to it as "a great run".
It is NOT a great run. It is physically exhausting because it is not on flat ground. If he had mentioned "Heartbreak Hill" at the 20 (?) mile mark, THEN it might be more plausible that he knew what it is like to run the Boston Marathon!