Fueling and doing a Century is simple...Follow the original long distance guru Velocio's Seven Commandments makes it relatively "simple" to do long miles:
1. Make your stops short and infrequent so as not to lose your drive.
2. Eat lightly and often. Eat before getting hungry, drink before you are thirsty.
3. Never ride until you are so tired that you cannot eat or sleep.
4. Put on extra clothing before you're cold, and take it off before you're hot.
Don't be afraid of exposing your skin to the sun, air, and rain.
5. Don't drink wine, eat meat, or smoke---at least during the ride.
6. Never rush things. Ride within yourself, particularly during the first few hours of a ride when you feel strong and are tempted to force the pace.
7. Never pedal out of vanity, don't be a show-off.
Without going into details on the Krebs' cycle, there are basically six sources of energy fueling a long ride that should be considered.
1. Stored glycogen within the muscles (maybe 1000 Calories in the legs)
2. Stored glucose with the liver (maybe 400 Calories...mostly to the brain)
3. Fat "converted" to energy in your muscles (mitochondria....virtually unlimted)
4. Amino acids "converted" to energy in your muscles (not good, who wants to lose muscle)
5. Foods that you eat during a ride (there is a finite limit to processing during a ride)
6. Ketones (very few riders have developed this source of energy)
As a distance rider gets fitter, they rely less and less upon the glycogen stored within the working muscles by using fat (beta oxidation) and this is a good thing for two reasons. First, most of us have limitless stores of fat for all practical purposes. Secondly and much more importantly in my view, oxidation of fat has a higher yield of ATP compared to burning glycogen (gluconeogenesis) and fat burning generally results in fewer H+ ions hanging around although with training, lactate is being recycled more efficiently plus H+ concentrations are lower than in the untrained state. So, it is kinda complicated. Riding just a little slower can very significantly tip you toward burning fat and a much greater time to exhaustion. Finding that pace is important. I've spent a lot of time reading and applying what I have learned and have ridden nonstop for more than 48 hours straight. What a rider eats varies a lot but one thing is certain, successful riders at longer distances eat and drink a little bit frequently. For instance, fruit does not work for me. Fructose makes me nauseous. YMMV. Some riders like to eat "real food" such as a hot dog or PBJ or chicken nuggets. 100% liquid works for me. I use First Endurance Pro in my bottles when I can afford it and make my own powders using highly branched cyclic dextrin, maltodextrin, D-Ribose, BCAA, and some minerals most of the time because it saves a lot of money although my brew does not taste as good.
The main reason to do longish training rides is to build fat burning capability because it is impossible to replace all of your glycogen once rides go beyond a certain distances and therefore, you must burn fat or bonk and hopefully spare the burning of your muscles. Stopping for a long rest also recharges the batteries if a rider bonks (depletes liver glucose and the brain becomes very unhappy). None of this matters much for short rides (say under 3-5 hours depending upon your RER) but fueling and the ability to burn fat becomes critical at around 6-10 hours of riding unless you want to feel really lousy. For the typical criterium rider, these long miles are junk or counterproductive because their success is mostly determined by the ability to laydown a lot of power for 5-8 minutes during crunch time and their races are short. But someone wanting to do a Century should be able to do 60-65 miles in training and if they can do that, a Century is almost in the bag.