Congrats, Jolt! I have never done the D2R2 (it typically conflicts with something else on my calendar), but have done PLENTY of NE gravel road riding/racing. In fact, my first gravel race is coming up on March 29, and then another on April 11!
The big thing to consider will be tires and pressure. Put the widest tires you can manage on your bike, and run the highest pressure you can. D2R2 is notorious for flats! So that said, make sure you also carry extra tubes (many people only ever carry 1, but on gravel rides, I will typically carry at least 2 tubes and a PUMP). The 100K will be doable, but be prepared for some pretty decent elevation as well. There are plenty of great training grounds in ME with both gravel AND elevation, and you have plenty of time between now and then. I typically spend some good amount of time in Bethel in the summer, so would love to connect for a ride at some point over the summer if you are interested.
Nutrition and hydration for me in these races isn't necessarily a big issue, but I don't run the norm (that is, right now, I can go out and ride 3-4 hours and eat 1/2 a Clif Bar and less than a 1/4 bottle of water). There are usually good rest areas where you can refuel on all of these rides, and you usually also occasionally pass a store where you might be able to purchase food/drink if you need it.
The most important, IMO, is to connect with a small group of similar riders and to share the experience together. In big rides like D2R2, this selection often happens naturally, and you will make some great friends while you enjoy what I've heard is beautiful scenery, and an awesome ride.
SheFly