From talking to other marathoners and their families, if you head to the park first thing in the morning, it'll be pretty empty. The 1/2 doesn't end until 9:30, the full marathon not until closer to noon (for all runners to be in). You might see some traffic problems with the route, but probably not many, esp. if you're staying on property. The route is posted on the disney world of sports page. Just click on the marathon and follow the links.
Ashley
(first time runner of the 1/2, staying at OKW)