Hmmm well, by mile 24 I was pretty much just focused on just getting through the race, but I'm pretty sure that the path going from Boardwalk to HS was closed to non-runners. I'm not entirely sure about the International gateway area of Epcot, if that was open to non runners? But in both parks, there are areas where the runners go through and they will have cast members with ropes to keep the crowds back. They actually have a really great system where they are able to move the ropes and change the course so that crowds can pass--so they will do that every few minutes; but if you're just part of the crowd you may have to stand and wait.
If I was going to a park on marathon morning, I would choose Magic Kingdom because that is the first park the runners go through (race starts at 5:30 am) so the runners are out by the time the park opens in the morning. Animal Kingdom would be my second choice because that's about the half way mark of the marathon, so it will clear out of runners long before HS or Epcot do.
Just to clarify, you're talking about the full marathon in January, right? There is also a half marathon the day before, that spends some time in Epcot.