Everyone's making such a big deal about Chicago's weather predictions. I've run the last 12 Boston Marathons, and I've seen some bad conditions. Try training in New England through sub-zero 20-milers for 2 or 3 months, then arrive at Hopkinton on Marathon Monday looking at an 80- or even 90-degree run in April. Before the recently-instituted 10 AM start, it was at high noon with no leaves on trees for shade. In the '04, '05 & '06 marathons, we all sweated our asses off.
My point is this - adjust. Run conservatively in the early miles, gauge the way you feel, and adjust. Don't expect a PR and run it feeling proud that you will probably be part of a record-breaker for conditions.
BTW, the second-hottest ever Boston Marathon, plus-90 in 2005, was a slow one for me but I hydrated adequately and recovered in 2 days. I went on to have one of my best racing seasons in 5 years in the following months.
Good Luck 2 All!