I'll be going down for my third marathon weekend, and the parks are always surprisingly slow during that time. Disney schedules all the races during off peak times, and the racers don't make that big of a difference. However, since Sunday is the marathon, you might want to look at your park schedule since pathways will be blocked for the racers in the morning, particularly at Epcot. Last year, they still had the ropes up at 1 and were signaling people around the runners.