Whorty, I can agree with Bridgy's comments about the physical endurance on longer rides.You have done the first major step and booked your ticket! I have not done many, and probably wont do any more. I trained to do my first 100 miler a few years ago, building up distance to around 90 miles. It took several months(4 actually) of weekend rides to go from easy, comfortable 50 milers to make the jump to 90 miles. It takes time for the body to adapt to long hours in the saddle. I personally found my Achilles became sore after 4 or more hours in the saddle, a tweak to my seat height of 5mm relieved this. Once I had done one 100 miler, mentally it was easy to another then another. The DD with 12K of climbing is quite a task, I would personally alter my gearing to allow easy spinning up long climbs(I have 40T cassette for silly climbing, big guy setup

).
A side note, I set up a mates bike with a 34T cass. He wanted to do the '
Wiggle punisher 100 mile' 10K feet climbing. He was relatively new to cycling but could do 70mile club runs OK. He said it was the hardest exercise he had ever done, unfortunately the organisers diverted his route down to 70 miles because he was too slow.
Don't go too hard or follow anyone above your comfortable cruising pace, or you will pay the price later on. This pacing is crucial to enjoying the event, you will find a pace which allows you to ride mile after mile after mile during training. Good luck and we look forward to yours and other comments of the day