Depends on the type of long run you are doing. If just an "easy" long run to get in the mileage and build endurance I wouldn't worry about it too much. Anywhere from 7:15 to 8:15 is probably fine. Depends on whether you typically run your easy runs on the faster or slower side. I have run sub 3 and typically do my long easy runs between 7:40-8:10. Slower side early in training and faster side late in training. Take a look at the McMillan calculator for a range. For long runs at goal marathon pace you likely know which pace you want to be hitting for those portions - probably right around 6:50.
Chicago is a flat, fast course with lots of turns. Second half is more out in the open where you are exposed to the elements and will feel the sun if it's a sunny day. For your marathon pace training runs try and do them on a flat course with lots of turns and out in the open if you can.