The first thing that you need to understand is that this is a Disney race, and you are going to be doing a lot of standing around. Traffic on those mornings is an absolute nightmare, so I'd suggest getting on the earliest bus possible....it gives you the least amount of hassle. You'll go to the family reunion area where a DJ will be entertaning. There have also been food trucks there if you didn't grab something to eat.
If you are checking a bag, you will also do it in this area, and use the port-a-john, which can be a 20+ minute wait.
Once they call for runners to go to the corrals, there is about a 1/4-1/2 mile walk from the reunion area to the actual start corrals. There is no away to avoid this walk. The crowd all moves together as a big herd.
When you are in your corral, there is plenty of start line entertainment, so time goes quickly.
We have always been on the first bus at 3:00 am. We've seen too many people sprinting to their corrals just as the race is about to push off because the buses got caught in traffic.