This article is from the Misc Bicycles FAQs, by various authors.
You will need to eat A LOT on the ride.
If you are doing an organized ride, they may have rest/food stops with
food and water. Check ahead to find out what kind of food they will
have and how frequent the stops are. Food offered might be fruit,
especially bananas and apples, cookies, peanut butter and jelly
sandwiches, peanut butter crackers, fig bars, and bagels. Some rides
will have sports drinks like Gatorade or Exceed. You may have to
depend on stores or carry a lot of your own. The above list is a good
list of things to put in your jersey pockets or in a handlebar bag if
the ride is not supported. (Jerseys (with pockets along the back) are
great for carrying food.) High sugared candies may give you a temporary
boost, but they will also let you down soon after. See the FAQ on
nutrition for more details.
Eat before you are hungry and drink before you are thirsty. Carry at
least two water bottles, and refill them regularly. Getting dehydrated
is one of the most common problems on long rides.
And just let me repeat this for emphasis. Eat Before You Are Hungry
And Drink Before You Are Thirsty!