Some great helpful advice above, I’d say as suggested above do the 65 miler ASAP, get that one in the legs and mentally your there. You can then concentrate on the little things like when you get hungry, thirsty, speed and pace, ride in a group before if can maybe at local club level.
I did that a few years ago and was hooked immediately, the buzz from riding in a group at pace on closed roads is addictive, what you don’t get sorted out between then and now will be absorbed into the thrill on the day and that’ll see you right. Just enjoy the riding between now and then, keep track of your performance and monitor your times a tad.