Did they ever actually say it was the NYC Marathon?
I don’t know for sure. But were there any other marathons in NYC during the 90s? The Brooklyn and Queens marathons were not around in the 90s, and I doubt the Seinfeld gang would travel to a different borough to watch a marathon. In addition, in the episode, it appears they are at mile 25 and in Central Park. So I have no reason to believe it isn’t the NYC Marathon.