We went this past Jan. 4-12 and it wasn't that crowded. MK was a bit crowded one day we were there, but it was also in the 80s that day! The rest of the week was on the cool side, so no crowds at all. Our resort (POR) was busy towards Marathon weekend, Marathon morning there were lots of crowds and runners on the road, and Marathon night DD/PI was busy, but the parks weren't that much busier. It was actually neat to see all the runners, then all the tired runners with their medals.
HTH!