As others have said - eat & drink little & often. Drink before you're thirsty & eat before you're hungry.
This is what works for me:-
The night before a long ride, I'll have a pasta-based meal.
Breakfast on day of ride will be an extra-large bowl of porridge topped with fruit and *lots* to drink to ensure I'm fuelled and well-hydrated before the ride starts.
For the ride itself, find an isotonic drink you like & take more of it with you than you think you'll need. One tip is to keep a sachet of the isotonic powder so that if you stop for a bottle/glass of water somewhere, you can make up a bottle of isotonic. That way you'll keep replenishing the salt you lose and get some engery into you too. What works for me is a citrus flavour isotonic drink. I also find energy gels incredibly useful and absolutely adore
these in raspberry flavour. I take a stack of gels with me on a long ride and take them regularly. I also like to have a supply of energy bars; current favourite are
SiS bars in cherry & vanilla To vary I'll also use malt loaf, flapjack, dried banananananananananananana, dried apricots, Torq bars in raspberry & apple or dried bananananananana
At the end of the ride, have some protein as it aids recovery. There are special recovery drinks you can buy:
this is the favourite of my offspring - chocolate flavour
Hope this helps.