The marathon does affect the crowds in two ways. The crowds are up, but still in NO WAY busy, just more than any other time in Jan. The other way it's affected is the day of the marathon. The runners run through all of the parks. Due to that, they change some of the bus routes making bus rides a little longer. Also, because they run through the parks, you may have to dodge them when first entering the parks. Also, we had a lot of trouble with all the family & friends using the buses to catch up with the runners, so it was always standing room only on the buses.
I've been twice during the marathon weekend. It not bad if you plan for it. The first time we didn't & we were sorry. The second time we specifically stayed at BWV & just took the boat over to MGM that day. It won't be bad, but it will affect the crowds, just in a different way. The good part about it is the ride lines are empty the day of the marathon, but not the days leading up to it or after.