The roads are very wide, so you shouldn't find much in the way of crowding.
I was in an open corral, but then I'm not fast enough for anything else. If you put yourself right at the front of the open corral, then maybe you should be ok.
Have a look on the US version of RW, as there should be Chicago thread going on there. I used it last year, and they are a friendly bunch. Even made it along to the post marathon booze up in the evening