In addition to all the good advise above, here is an easy way (for me) to do a century.
Start very early in the morning, after an Oatmeal Breakfast. Do 25 miles and eat something good. (I like Subway sandwiches) En route drink three bottles of water and Gatorade. (50/50)
Repeat 25 miles. (do not forget hydration)
Have a nice relaxing lunch with sensible food such as pasta salad, peanut butter/jelly sandwich, honey, etc., or another Subway.
Go for a little walk. Take it easy.
Repeat the above for another 50 miles in the afternoon.
I use a fair amount of trail mix to get quick energy.
My wife and I do vacation trips using above style, on a Tandem. No problem and we are 65.