Actually, consider the following:

Your dates coincide with two major meetings: The Neurosurgeons Congress and the Graph (Graphics) Expo. Also, an even bigger medical meeting, the American College of Surgeons, concludes on the 4th (and there are sometimes private events after the formal show, especially in the medical field as the pharmaceutical companies take advantage of all those doctors in one place at one time)

Medical conventions tend to have fewer cancellations, as there are continuing education/training programs attached to many such meetings.

The Marathon just makes the rooms situation even tighter, but the race is not the main factor.

So if you can't change your dates, one alternative you might look at is staying in an outlying area (like the O'Hare area) and commuting in on the CTA.

Best wishes in your search for accommodations.