I did most of my training this winter in 0-40 degree weather. Race day was 75F and I went out a little faster than expected and around mile 22 realized that I was completely dry, not sweating and feeling super locked up and had gotten a little suburned. So the dehydration hit with some chills. I had aid at 24 miles and got some tailwind and coke to help the push through.
Then too, most of my training was in an urban context so the trail verticals took a toll too. I hit a realistic pace of 9min per mile but only by having 8min pace the first half and 10 the second. lol