The marathon is on Sunday and starts at 6AM from Epcot. The runners have up to 7 hours to complete the course. Route is: Epcot, MK, AK, WWoS, DS and back to Epcot. A section of each park is part of the course, so you will need to watch out for the runners.
On Saturday, the 1/2 marthon will take place in Epcot, goes to MK and back to Epcot. Runners have 3.5 hours to complete with the race starting at 6AM.
The 5K race starts at 10AM on Saturday and goes through Epcot, mostly the World Showcase, but finishes past Spaceship Earth.
The times are different from the last years when the marathon and 1/2 marathon were on the same day at the same time.
Park crowds are not that bad. Just watch out for the runners and don't cross in front of them.