Training for a 100 miler is no more than your normal riding. No need to 100 miles training for a 100 mile ride as you will have already done it. If you can do 60- you can do 100 with a bit more training. Just modify the training. 50 to 60 at weekends and a couple of rides of say 20 miles in the week but make those 20 count. One ride on a flattish route but with a bit more speed than you normally ride at. The other up hills and I don't mean slopes. If more mileage can be done with extra rides then all the better.
A few problems that you will come across on the 100 and one is butt ache so saddle time will help. Nutrition and Hydration are a major concern so carbo load beforehand- have a good breakfast and drink and snack right from the start of the ride. I normally only take one break at around the 65 to 75 mile mark other than for watering stops and at that break I drink a bottle of energy drink- eat a bit more substantially and take a stretch to loosen the muscles. That break serves two things- Besides the rest and nutrition- it stop you finding the wall at around the 70 to 80 mile mark.
Only other point is not to over exert yourself by staying with a faster group and don't slow yourself down riding with others where you will be doing most of the work to pull them a long. Find a group of your speed so you can draft with them to save energy and just enjoy the ride.
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How long was I in the army? Five foot seven.
Spike Milligan