Re: Chicago Marathon - please pace me!
With you having done 5 previous marathons, I'm surprised you don't have your pace nailed by now.
What I'll offer is to not get too caught up in pace and instead go by feel. You know what pace works for you, and you know what pace doesn't. If you think you can go out with the 4:45 pace group, do yourself a favor and go with them. The group will help pass the miles before you turn inward, and the longer you can put off between turning inward, the better chance you have of holding on -- if you paced yourself.
But when you go out with the pace group, if the pace feels too fast, then let them go. But don't do it the first time you wonder whether they're going too fast. If you know for a fact they're above you, let them go. But if you're not entirely sure, stick with for a while longer and re-evaluated. I suggest this because of the point I made earlier, which is it is easier to run marathons and finish strong when you can get with a group to take you as deep into the race as possible, because once you are left by yourself, you will be tested greatly, as you rightfully know by now having done 5 already.
The gist is, trust yourself and your body. Go out with the 4:45 pace group if you think it's a nice fit. If at any point they are too fast, let them go -- but only once you know they are burning you out by going at that rate. Then assess constantly and adjust. Just like you would in triathlon.
Furthermore, do yourself a huge favor and stay on top of hydration and nutrition. You will be on the course for nearly 5 hours. That's a long time. You must get in calories and stay on top of it. You can fake it to the 3 hour mark, or maybe beyond, but then it'll catch up to you. Never let it catch up. Gobble Gu's or your favorite calorie source at a steady rate. I'm often amazed at how little water/Gatorade and few Gu's marathoners take out on the course. Most of these people come across with sure signs of dehydration and near or having bonked, and the thing is, they think they had enough. I never realized what their problems were -- because I believed them -- until I started doing long course triathlons and kept up marathoning. Now it's obvious as ever.
Trust your instincts.