The pace for the first bit is ideally that of your corral placement. So, the theory is that you will be flowing at relatively the same pace as your corral mates. If you've got a fast game on, be wary of that fast start. We all tend to go out too fast and will burn out unless we are mindful of our pace.
Back of the pack is congested, but I find myself getting my pace on rather quickly, big thing for me back there is to keep it at my comfort level and to pay attention to not going too fast too quickly.
There are folks in all of the corrals that for whatever reason will have injury, or a bad day. The congestion will take a few miles--until after the merge (after Epcot?) to sort itself and you will have some better on-pace stretches. The miles are filled with weaving and as the roads widen and narrow along the entire route there will be bunching. You should have a decent pace early where the road is wide and hopefully back of the pack-wise be at the pace you want.