I received my first email newsletter today from the Chicago Marathon and was shocked to learn that there will be 45,000 registrants for the race. Wow! As a point of comparison, the New York City Marathon in 2007 had 38,761 finishers. It's hard to imagine what it will be like to be at the starting line with 40,000+ other people; I think the biggest race I've been in so far had under 5,000.
The official Chicago marathon website has a countdown timer to the start of the race. It currently says: 172 : 17 : 11 : 41 (days/hours/minutes/seconds). There's a lot of running to be done between now and then - something like 650 miles!