I recently did a Manchester to Liverpool ride which was 40 miles. It was in the heat and the last stretch was a bit of a killer but we didn't really take the breaks we should so that's the main advice I would give. Every 10 miles or so and take a 10 minute break, don't try and complete it in a set time. There will be checkpoints if it's organised and they're there for a reason so stop and take a breather, get a drink etc. A Camelbak is a good shout, they're expensive but worth the investment. You could make one yourself by getting a cheap ruck sack and the bladder from EBay.
Get padded shorts and get some gloves, gloves will protect your hands from the elements but also prevent you getting blisters especially over 100 miles
Is it off-road, on-road, mixture? That should determine what bike you get.
I have a Marin Northside Trail
for off-roading which my father in law borrowed for the Mcr-Lpool one but I wouldn't want to do 100 miles in that, it would hurt a lot more! You would be better with a Hybrid, I have a great Hybrid called a Scott Sportster
which I absolutely love and is perfect for the types of rides that Bike Events offer - mixture of on and off road.
I have bad sciatica and arthritis which is now in my neck so would really struggle with anything more than 40 miles but I'd love to do 100 miles. Enjoy