I was a little curious, so I headed over to the Disboards forums, and I did see references that getting a better block is pretty important because there are a lot of bottlenecks.
Looking at the course route, I can see what they mean–you will be running through three theme parks, and while you won’t be dodging strollers and wheelchairs, the pathways are still going to be closer to “alley” in width than “major city street” at times.
Looks like you hit the gates to Animal Kingdom a little before mile 4, which doesn’t seem like enough time for things to stretch out at all. After circling around there you head over to Hollywood Studios, then up the sidewalk to the boardwalk area and into the back gate at Epcot.