The race crowd does little to affect overall crowds, there are about 20,000 runners, and not all even stay or go to the World while there. You maybe slowed down by the race in the mid morning hours as the back half of the runners will be in AK and MGM and Epcot while these parks are operating. The median finishing time is over 4 hours and the race starts at 6AM. The greatest lump of runners will be in AK between 8:30 and 9:45, MGM 9:15 to 10:30, and Epcot 9:45 and 11. The race course is open to runners until 1PM I believe, and the course is marked in the parks with yellow cones. You will be asked to avoid walking in the course area, but are free to cross the course when appropriate.
If you are interested the men's winner will cross the finish line at Epcot around 8:20-8:30 and the women's winner will cross around 9.
Its a very cool time to go and you will see the marathon runners all wearing their finishers medals for the rest of the weekend. If you aren't sure, just watch them walk, they look pretty sore and they move pretty slow!