When we were there this past January, we were there from 29 December - 6 January. We thought that the crowds would die down after New Year's Day. The marathon was the next weekend and the resorts and parks were packed.
We heard some of the runners, on the bus to EPCOT, saying that they had gotten package deals. They decided to bring there families since they got package deals. The parks were crowded the whole time we were there, but we did have a good time.