For Chicago, I like to go to a track about 3 times over the last two weeks. I'll run 2x2 miles at marathon pace, or a 2,1,2 at marathon pace. The first time I'll look at my watch every 400 meters, then the second time I go to the track it's every 800, then it's every mile.
The start of Chicago will be very fast because it's flat and everyone's in eager anticipation of the day. It's so easy to get sucked into running a pace that's 25 seconds faster than what you want in the first mile. You need to be able to lock into that first mile pace you want, otherwise it'll really come back to haunt you in the last 10K.
Of course you can run any workout you want, but you want to run a few miles on a flat surface and be able to lock into your pace from the getgo.
GOOD LUCK !!!!!!!!!!!!