So he lied about it to sound more impressive, even when talking of totally unimportant accomplishments. Even though no-one actually cares about your marathon time other then your running buddies (if you have any). Pathetic really.
And he said marathons plural in the interview, when it turns out that ran just one. Anyone who has run a few marathons can remember them.
It's all part of the GoP campaign to reach out to certain demographics ... in this case the Pathological Liar demographic.