I live in GR and ran the marathon last year.
It's a very well-organized event, and the traditionally easy course has been slightly modified this year but is still very fast.
This is what I see as far as elevation changes:
during mile 2 (in the downtown area), the course climbs from about 600 to 620ft;
from miles 7 to 9 (on Butterworth), the course rolls up and down, going from about 600 to 650ft - this is the toughest part of the course and it's not tough;
during mile 10 (heading to Millenium Park), it drops gradually back down to about 600ft;
after that, it's all between 595 and 610 ft with no substantial inclines or declines
The toughest part of the race will be that you will be on your own for long stretches, esp at your goal pace. There will be no spectator support because the second half is almost all along the river and it's difficult or impossible for people to get close most of the way (the turnaround in mile 18 being an exception). Water station support people and runners going the other direction will be your main human contact. Although most of the course is very scenic, the last 2.5 - 3 miles aren't pretty and there will be extremely few people around (just the half-marathoners going at 12min pace), so it's a bit of a tough finish despite being flat.
I think it's an easy enough course, esp in the 2nd half, where you can certainly do a negative split. Maybe go out at your overall goal pace, plan to run a bit slower in miles 8 and 9, get back to goal pace and see if you have anything left so you can pick it up in the last few miles?
I ran 3:11 last year when I was undertrained because of injuries. This year I'm healthy and hoping to break 3:00. I'm doing miles 7 to 22 at my goal marathon pace on Monday - we'll see how it goes!
Good luck!