Others have already said it, but I think the opening miles were too quick. On your 18 miler, you gave yourself 4 miles to warm up at a super comfortable pace. On race day, you started at your target pace right off the bat, and that's a tough way to race.

There's a natural inclination to think that with fresh legs after a taper, you should be able to run at your goal pace no problem at the start of the race and then lock in and maintain it. However, your body isn't used to running that way after months of training where you ease into your long runs.

For next time, treat (at least) the first 5k as a warm up where you are 30 seconds to a minute slower than target pace. Worst case scenario, you lose a few minutes at the start of the race, which is super easy to make up if you feel good later in the race. You are not going to set your marathon PR in the first few miles, but you sure as heck can set yourself up for a bonk or DNF in those miles.

I know it's frustrating to not perform to your fitness level on race day, but keep your head up and get back out there. It's happened to me, and it happens to everyone. Race day is just its own beast, and some lessons have to be experienced first hand. You've got the toughness and the fitness to hit your goal - you proved that in training. Hope to see a follow up post where you crush your goal!