Just read through the thread and I think you already have the main bits of advice from others. However, I'll repeat a couple of what I think are the most important bits for emphasis.
Get your position sorted out now and do all the long rides that you can in it (as I was doing when you saw me using my TT bike in the Anfractuous the other week - I'll be using that bike for everything that isn't too hilly). As you get used to riding on your aerobars (they are a must) you will be able to drop the height down and bring them closer together - both of which will improve your aerodynamics, which is free speed / distance.
You may need to change some things, such as your saddle, to work with a more aero position - get all that sorted out as soon as possible.
Definitely do a 100.
It's worth doing some other TTs too, to get more practice and to work on your speed. eg Come along to Hillingdon on a Wednesday evening and do ours (10.3 miles on a circuit).
Make sure you have sorted out nutrition beforehand - what you can eat and that it fuels you enough - and test it out on long rides before. In fact, test everything on rides beforehand!
If you want to read something, then Simon Doughty's book on Long Distance Cycling is good. He suggests a training schedule for the 24.
Good luck!
