K.W.
The adrenalin that will come from being in the race, and the support from the other runners and spectators, is likely to help you overcome your reduction in training. If you've had an 18.5+ mile training run, I think you'd be fine.
Long distant runs are a mind game almost more than a physical one. It's about ignoring the aches and being a duracell bunny.
At around mile 21, pretend you are Dory from Finding Nemo and keep repeating, "Just keep running. Running, running. Just keep running. Running, running..."
Good luck and try to have some fun out there!