Crappy long run with less than 2 weeks to go...

Coach Brian wanted me to do between 16 and 18 miles as my long run this weekend. But because of other stuff I wanted to get done, I pushed off the long run until this morning (Monday). I chose 16 miles because I'll be riding and playing hockey later today, so the muscles will definitely have an '18 mile' workout today, even if I just do the shorter distance. And my plan from the start -- really, really -- was to do 10 miles at long run pace (8:20/mile), 5 miles at marathon pace (7:50/mile), and a 'cool down' mile at long run pace again. Here are my splits:
8:34
8:15
8:13
8:15
8:13
8:09
8:13
8:13
8:15
8:14
7:50
7:48
7:44
7:48
7:47
8:18

So, on paper it looks okay, right? But I was dying out there. The weather was much warmer than it has been recently -- between 15 and 17 degrees C -- and quite muggy. The last 6 miles I did with no shirt, which I haven't done since the summer. And when I stopped, I felt like I was going to puke (at general sign of overheating for me).

And I felt tired the way through the 'marathon pace' miles.

So here come the doubts: I can't do this for 26.2 miles! But, having been there before, I know that it will all come together. A couple of weeks of reduced mileage and a bit cooler weather and I'll be ready to go. Just sucks when you have a run that causes these doubts so close to the marathon date!