Just note that Marathon Weekend is that next week-end. There's a 5k at EPCOT on Friday, Jan 7, a half-marathon on Saturday, Jan 8 and the full marathon on Sunday, Jan 9.
There will be some road closures and delayed park transportation on Jan 8 & 9 due to the races. The half marathon should wrap up around 10 AM on the 8th and the full should wrap up around 1:00 pm on the 9th. The parks are pretty dead while the races occur but after theyll pick-up quite a bit or at least last year they did. The races go through the parks (half MK (before opening) & EPCOT; the full- MK (before opening) DHS, AK & EPCOT) so youll have to yield to the runners in the parks. Theyll have things marked off with cones.
Monday, Jan 10 is usually the day that people show up to Magic Kingdom wearing their medals. The MK was packed last year on that day. This year, to encourage more people to sign up for the full marathon, theyre offering free admission into the MK on Jan 10 if youre wearing you 2011 full-marathon medal.
So, I would think, right as the NYs crowd is leaving the marathon crowd is arriving. I wouldnt really expect things to die down until Jan 11.