The sweepers are behind the last person to start. As long as you can do a mile in 16 minutes and stay ahead of them you are good. The best advantage is to be farthest up in the corrals so you have a time cushion. This year the corrals went from A through H. So the faster you can go the closer to the front you start. I usually can run between a 11:30 and 12:30 minute mile, but with stopping for pictures my pace turned out to be something like a 17 minute mile. But I started in Corral B, so I never saw the sweepers. Check out on the WISH board there is the Official Princess half Marathon thread. Everyone there is so friendly and supportive. And there are lots of people who walk the whole thing, some who do a run walk combo. and those who run from photo stop to photo stop (not super fast, I have to say none of us are Kenyans!)
ETA: Kinda like Jud said, but by the time I got to Epcot the park was open. I think they try to get all the runners through MK before it opens.