The 1/2 marathon starts at 6AM on Sat. The runners have 3.5 hours to complete the course, so they have to be through the MK by 8AM at the very latest. Following the 1/2 marathon on Saturday will be the 5K race at Epcot, goes mainly through the WS, but does exit past Spaceship Earth to the parking lot.
The full marathon takes place on Sun. Again it starts at 6M. Runners have 7 hours to complete the course. Starts Epcot, goes to MK, the AK, then WWoS, then Disney Studios, past the YC/BC/BW, into Epcot at the International Gateway and finishes at the parking lot.
When the runners go through the parks, they are not really roped off. You just have to keep out of the way of the runners. Don't cross in front of them. Sections of roadway will be blocked to vehicular traffic for the runners. Try not to plan a Chef Mickey's breakfast for 7:30 or 8AM either morning. You will have a hard time getting through unless you can get to the resort by monorail from Epcot.