The typical terrible last long run

It's what I do: have a terrible long run right before a marathon.
Saturday morning I just messed everything up. I had my last 20-miler on schedule, and I made sure it was awful. I overdressed (again) and ran by the house at mile 4 to toss my shirt. I was also already feeling wobbly. I have this annoying insulin-dump thing in the mornings. If I eat before 7am, my body responds with a huge amount of insulin, and I quickly feel faint. If I wait until after 8am, I don't have that response. So I usually have a late breakfast, and I don't eat before running unless it's 18 miles or more. At that point, that stored glucose might still come into play at the end of the run, so it's worth struggling through. But today's half-a-plain-pancake (the cupboard is bare over here) left me weak and hungry.
Brilliantly, I grabbed a Honey Stinger waffle as I went out the door. I'd never eaten one before, but I've heard good things, so I bought some a few weeks ago. However, when I read the nutrition label, I saw that there were 7 grams of fat per waffle, and I knew that my stomach rebels if I have high fat while I run. For whatever reason, I chose to ignore that fact on Saturday, and I chomped the waffle as I ran.
Almost immediately, my run started to suck. For one thing, I got to the levee to start the hill repeats that I've been doing in the first half of my long run, and there was a train blocking my way. And it was stopped. I ended up detouring all the way through the park, and by that time the train was gone, but I also pushed my hill repeats back by about 4 miles, which can be tiring. It's hard doing hill repeats in the second half of a long run.
And by the time I finished my last hill repeat, my stomach was in knots. I felt over-full (a Honey Stinger waffle is a lot of food, and in addition to the 7g of fat, they have 160 calories - that's a lot for early in a run) and a little ill. I knew I needed to start adding more fuel, because I was at mile 12, but I couldn't stomach anything.
Finally, at mile 15 I opened a gel, and immediately made a beeline for the bathroom with terrible stomach cramps! That was the end of that gel and any attempts at fueling for the rest of the run, which meant that I was just hanging on for the final miles. Walls-ville. I still squeaked out 20 miles at a 7:50 pace, but I think that had less to do with feeling comfortable at that pace and more to do with
1. A fifteen-minute break in the bathroom, and
2. A strong sprint home...to the bathroom.
The entire rest of the day was spent doubled over with incredibly painful stomach cramps. I curled up with a hot water bottle and watched the Olympic Marathon trials and wished I was dead. It didn't help that I couldn't eat or drink, even though I knew I needed some fluids and electrolytes: not to mention food! I finally wised up and took an Imodium, and I gradually started to feel better.
Lesson learned: No or low fat for running, and I am very reliant on carb fuel at this point. I wish I was a better fat-burner, but I don't have the training miles needed for that right now.

So that's the story of my wretched final long run. I finished, it wasn't pretty, and I am not expecting miracles for the marathon coming up. Realistically, I think I can do a sub 3:30, but I'm not so sure that 3:20 is a reasonable goal right now (Ouch. Don't get old and have surgeries, youngsters). Thoughts? Predictions?