We arrived the day before the marathon in 2004 and the day after the marathon in 2006, neither times were what I considered crowded, its actually our MOST FAVORITE time to go. The day of the marathon, the runners took a bit longer to get thru the MK but we had not problems with rides, once the race was over the runners go back to their hotels for a nap. This was a bit hard, keeping mykids quiet in the halls, most doors had DO NOT DISTURB signs on them.
Have fun