Based on the circumstances of this situation, why would this particular individual cheat? It does not make sense.
It doesn't appear to be for attention, as this runner received almost none. He could've informed newspapers/media outlets and created press releases that, given the gravity of the time run and appealing nature of the content (old guy crushes marathon time and defies aging), surely would've been picked up. That doesn't appear to have happened.
He didn't seem to bring a cheering section to the race to inflate his ego, or post a bunch if stuff on social media.
Further, he seems to be living a fulfilling life with a successful practice, volunteer work, and coaching. Doesn't seem to have any type of financial motive to cheat.