For my first two marathons, I was not trained very well and did a lot of walking in the 2nd half. My 3rd was the first time that I ran the whole thing. I remember the pain in my legs after mile 21-ish was unlike anything I had ever experienced. But I also remember thinking how crazy it was that I was still able to run my goal pace, even with the pain in my legs. That's what good training will do for you.
You're very well trained and ready to roll. As long as you run the first half at the correct pace, you'll be good to finish strong. When it gets tough, dig deep and trust the training that you did.