OH and I did this one a few years ago and it was a blazing hot day, the previous year it had been snowy, so the weather can be very unpredictable. It is a wonderful event.
The jetlag will work in your favour! You will be jumping out of bed ready for brekkie while the locals are bleary eyed
. Just make sure you stay hydrated on the flight.
The course was very enjoyable and the support amazing. Chicago really embraces the marathon and I don't think there were any dull sections of the course. Memorable highlights were the 'military fairies' and drag queens dancing and singing in Boystown, college students offering beer, margaritas etc in the student district, the many many houses with music blasting out and parties happening outside.
If you want to turn it into a two centre holiday, also worth thinking about the trains. Chicago is the main Amtrak hub and we booked a sleeper train to Washington after the race. Amazing scenery, good food and interesting company - the train went through Pennsylvania, so quite a few Amish travelling. Pretty reasonably priced as well. We are planning to do Chicago again in the next few years and will do the sleeper train to Sacramento afterwards, to make it an even more adventurous break!