Eat a highly digestible meal the night before. Big struggle for me is my toilet times, I am very regimented. Eating steak, eggs and a few chips meant no 9am sit down.
Hydrate well day before and bearing above in mind keep full on things like seeds and trail mix.
On the day itself cliff blocks are a godsend if you get on with them but I found after 70 or so miles I wanted something more food like. Had chucked a small protein bar and a cliff bar in my pocket at start and they got me to the end.
Work up to it, 50, 70, 85 miles etc. before if you not already, and ride those at the pace you want for the 100. The only difference on the day is then a little more fuelling.
The ride is very flat apart from 3 or 4 places. I found this brought on back pain I had rarely experienced on any other rides, even longer ones. Make sure you stretch every little while and remember 5 mins off the bike if needed is fine if you are hitting target averages
Go and spend a day over the Surrey hills before the event. Plod up Leith a few times and realise that is the only physically tough part of the ride. I slowed right down up it on the day(s) and kept myself thresholdish. With the bike traffic that is a 10 min climb with a load of rest down other side.
Your start time is not something you can choose and it does make a huge difference. That said try and latch onto groups moving through even if you have a little push to get on. Once on, assuming group is the right pace, it becomes a lot easier.
My top tip is do not drop a water bottle in the first 5 miles. Couldn’t find a replacement and had to stop a few times to refill, when original plan was 1 stop.
You will go a lot faster than expected but enough training, fuelling and pacing and it is doable.