You sound exactly like my situation. I also ran Illinois last weekend. I was going for sub 3 (PR was last fall at TCM 3:06:20). 4-5 weeks before the race I PR'd at 5k and HM 18:03 and 1:24:30. I thought that sub 3 was definitely in the cards given how flat the Illinois course was.
Taper didn't go too well, I hurt my foot 11 days out from the marathon and only ran a few times before the race. The day before I tried to run on it and it hurt, but I went ahead and ran anyways. Thank god, no injuries, but the race was no fun with the rain and the wind. I ended up with 3:19:xx going through half in just under 1:32.
Anyways, after some thoughts, all that I can come up with for my poor performance was that I lacked marathon specific work and that I was running less miles in the buildup to Illinois (70 MPW avg 4 months before TCM and 55 avg 4 months before Illinois). I put in 7 runs of 18-27 miles during the 4 months so it wasn't a lack of long runs, but more probably about my foot and that I didn't practice marathon pace enough.