Generally speaking ...
If you ride less than 2 hours, you don't need to eat anything over and above what you normally eat.
If you ride approx. 2-4 hours, you might want to consider consuming somewhere round about 100 cal/hour while you're out there.
If you ride more than 4 hours, you might want to consider consuming approx. 200 cal/hour.
All depending on conditions, of course ... and how fit you are.
On a tour, we usually have a fairly decent breakfast, then stop for lunch somewhere, and have a decent dinner. Plus some snacks in between.