The half marathon and Family Fun runs are on Sat, Jan 7. The half starts at the Epcot parking lot at 6AM. Traffic will be horrendous at that time. Runners have 3.5 hours to complete the race which goes to MK and back to Epcot. Family Fun runs start at 10:30, I think. The 5K goes into Epcot around the World Showcase and back out to the parking lot. Children's runs are after that in the parking lot.
On Sunday, the full marathon starts at 6AM at Epcot, goes to MK, then AK, then Disney Studios, past the Boardwalk, through the International Gateway and back out to the Epcot parking lot. Runners have 7 hours to complete the race. The fastest runners could complete the marathon in about 2 hours, 15 minutes. There could be about 20,000 runners for each race (except the 5K and children's races). So stretch them out along the route and you may have traffic problems.
Personally I wouldn't schedule CRT for breakfast on either of these two days. Getting into the MK for an early seating will be tough unless you are on the monorail.