As far as marathons that close: I know of peeople who can get away with it. They tend to be experienced, very light, and have quick, efficient strides.
On the other hand, I've never been able to get away with that (I've tried 3 weeks; my second marathon was slightly faster, but still slow (3:55 then 3:46). But I am large and I have a plodding, inefficient stride.
I guess it depends on how beat up you are from Chicago.
Good luck, no matter what you try.
ps: I am hurt that Bambi made a post without mentioning me. Sniffle....
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