not sure where I stand

Well, This was supposed to be my last long run before the marathon. My training plan said to do 20 miles. I was a little unsure about making my last long run 20 and leaving 6.2 on the table. Now, I am leaving 8.2 on the table.
I ran 18 miles two weeks ago. Last week I knocked my long run down to 10 miles. The 10 mile run felt smooth and easy.
My plan was to make this weekend’s run like a race simulation. The Marathon starts at 8 a.m. I wanted to get up around 7:30. Drink my water and hit the road. Of course I slept in a little later. I am taking the Friday before the marathon off from work so hopefully I will be rested. I got up around 8:15. I washed my face, brushed my teeth, and got dressed. For some reason I was really hungry. I made a toasted English Muffin with peanut butter. I rarely have this as my breakfast. I finally hit the road around 9:15. For about 7 miles I was feeling pretty good. I always bring a water with me on the road. At about mile 10 I started to feel stomach cramps. Side stitches creeped up around mile 12 and didn’t go away until I was done. The weather was mostly cloudy and around 70 but with high humidity. It was 93% when I woke up. The humidity was brutal. Miles 1-10 I was moving along at my typical 8:50 – 9:35 pace. Miles 10 -17 I was slower than 10 minutes a mile. That is much slower than anything I normally do. I didn’t realize this at the time but after uploading my run on the computer there was a clear point where I hit the wall. In high humidity situations the body just can’t evaporate quickly enough. This can lead to increase body temperatures and lots of other symptoms. I hydrated plenty. I refilled my water bottle two times I refueled with an espresso Gu energy gel. I just could not get into a rhythm. My feet were killing me and my left hip was cramping up. I tried to gut it out for 18 but I couldn’t even handle that. 17.5 was the end. Normally when I am feeling this way I say to myself if you stop now you still have to walk home. I didn’t care at this point. I walked about a mile and a half back home. I took my typical ice bath on my longer runs. Right now I am dealing with aching feet and a sore left hip that feels like it could cramp at any moment. Also, just general fatigue. After I got out of the ice bath and shower I fell asleep for an hour or so. I am starting to second guess this a little. I think I can still do it if everything goes well but what if it doesn’t. Now the question is do I start my taper as my plan dictates or do I try to get up to 20 miles. I am 3 weeks out.