If you can up your long ride to 75 - 80 miles you will be able to ride 100 when the time comes. Adding 5% - 10% a week is doable considering the amount of miles you currently ride.
The compact gearing is a good choice unless you have some big long hills to ride. Usually the compact will give you all the gearing needed where the triple will provide about 1 and 1/2 more easier gears.
Eating is vital for long rides when your efforts exceeds the amount of stored fuel, the energy in the blood, liver and muscles, which is usually about 50 miles or a few hours’ worth. Once you deplete the stored energy you get to a bonk phase which usually means failure. You should practice eating on your rides, even the short rides which will enable your body to adapt to drawing fuel from the consumed foods while on the fly and extending the use of the stored fuel.