@Hula Baloo,
All races start at 5:30 AM, but runDisney starts runners in corral waves, so the last runners may not start until after 6:00 AM.
Friday will be the Princess 5K, which runs only through Epcot. Runners should clear the park by 8:00 AM at the very latest. One of the Epcot parking lots is used as both the staging area and finish line. You MAY experience some traffic delays first thing that morning as buses to take runners back to their resorts will be in full swing. I would suggest Epcot on Friday, February 21st. If Epcot opens at 8:00AM that morning, there will be some issues with traffic and parking.
Saturday, February 22nd will be the 10K, which also starts and ends in Epcot. The course will leave Epcot, go out to the roads around Epcot, enter Epcot through WS, run over to the Boardwalk area then finish at the same place in the Epcot parking lot as the 5K. The park will be open as the last runners come through. I was in the middle corral and don't start until after 6:00 AM. There will be traffic delays around Epcot that morning. Avoid Epcot in the morning. Any of the other parks will be fine.
If you plan on visiting any other parks Sunday morning, avoid MK at rope drop and Epcot until at least noon. The Half course runs from Epcot to MK and back. World Drive will have lane closures and will be closed early in the morning from 5:30 AM to 8:30 AM, if I remember correctly.
Of course, all this is subject to change as the redevelopment of Future World make impact the final legs of the race...we run past Mousegear to the finish line.
Princess Weekend is either the largest or 2nd largest race weekend, about 30,000 - 35,000 extra guests that weekend with runners, families & friends and volunteers. If you've not made a reservation to stay on property, I'd suggest you do it soon. Parks will be crowded in the afternoons when runners and their friends and family visit. But will start to clear of runners in the early evening as race morning means an early bedtime the night before.