Pretty much that is it. If you don't have proof that you can finish at or under 2hour 45 min for the Half then you are usually put in the last corral. Hey there will be LOTS of people in that corral. Don't worry too much as there are lots of WISHer that enjoy being in the back of the pack and will keep you company until you feel the need to speed on by.
If you are worried about staying ahead of the sweeps then try to be in the front to middle of the corral because the sweeper time start when the LAST person crosses the start line, so will have a bit of a cushion. But it's kinda early to worry, so just get out there and train. If you find that you are faster than the time you put down originally then bring proof to the EXPO and they will change your corral.
Hope that helps you,
Tracy