Just a few days ago, I completed my 100th century (100 mile ride) and also my 175th ride of 100 miles or longer.
What do I eat on all those rides?
I have a decent dinner the night before ... probably a bit more than I usually eat, but not overboard. And I choose foods that sit well. Not too much dairy. Not too spicy.
I have a modest breakfast, aiming for about 500 calories. If the ride starts really early, it will just be Ensure or something easy to get down. If the ride starts later, it'll be toast with cheese and toast with nutella and honey.
And then I aim to eat at least 100 cal/hour while I'm riding. On the longer rides, I try to aim for 200 cal/hour ... or sometimes even more for as long as I can. What I eat is usually granola bars, cookies, salted almonds and things like that.
I will also aim to eat a meal about every 6 or 7 hours on really long rides, but on a century, it depends. If the century is just a casual relaxed thing, we'll stop for lunch mid-ride. If it's more of an event, we'll keep going and just snack along the way.
Later on the really long rides I reach a point where I struggle to eat so it is important to eat solid food regularly early on ... and then later I might have to resort to gels and liquid nutrition. But centuries aren't usually long enough for that.
On a metric century (100 km) ... normal dinner, normal breakfast, and maybe 2 or 3 granola bars.