@
Ariel484 hit it pretty square on the head. Thursday and Friday the runners should be well out of Epcot and back at their resort showering before rope drop. Saturday you may run into congestion if you have a MK early AM dining reservation or tour (7ish) as World Drive becomes single lane for a few hours. Sunday is more of the same at the MK, but then AK, DHS, and Epcot will all be open while runners are there.
The resorts are full from people coming down just for the race, for a short weekend around the race, etc. There are a bunch of runners who won't go to the parks after they run, but others will (albeit at a slower pace than normal). They anticipate 60-70k runners between the 4 events (that number does count each event separately, so those of us who do all 4 would be counted as 4 runners) with the half and full each having around 25-30k runners.
The most important thing? Pay attention to all posted signs and directions, and leave extra early Sat/Sun AM if you have definite plans. WDW will make you well aware of any anticipated problems via roadway signs, letters in room/at registration, and even voicemail left on your room phone. If you pay attention to road closures, and any anticipated time delays, you will be fine.