I'm not planning on doing it this year (although I am tempted to try it on a fixed gear at some point), but I have done it a couple of times before
The 100 is a great ride, and takes in some nice bits of norfolk. Be prepared for long slopes out round the coastal roads (for Norfolk that is) and certainly a steep climb near the start (it was Gas Hill (about 16% at it's steepest) the first year I did it then it was Kett's Hill (about 6% all the way) the second).
I really enjoyed both my rides, once on my own basic touring bike and the second time rotating with 2 other riders on a tandem and lightweight road bike.
There is a little support available to help stranded riders but I wouldn't rely on it; take repair kits etc with you, even if you end up not needing it you may be able to help someone else out.
What kind of distances have you been riding recently? I found my range is much longer riding in a group compared to riding alone, but I don't have anyone to ride with so I haven't done any real distance myself for a long time now. I hadn't riden more than 50-70 in a go before the 100, and the extra mileage didn't seem that much more challenging.
What bike are you going to use?