personally, i'd cut out the structured speedwork between now and Chi-town. i'd recommend doing fartlek style efforts to stay sharp and keep putting in the miles.
Actually, after reading this, I had a flashback to last year. I hit the training wall about a month or so before Chicago. I got off the track and did just what cameron is suggesting. I didn't get that sub-2:40 goal but I was only 6 seconds away and I gave up a lot more than 6 seconds with an unwise start.
Chris, this actually sounds like a lot of people I know. When trying to find your limits, you sometimes go too far. It's the nature of the beast. The key is to know when you did go too far and what you can do about it.