Drink a lot, eat a lot, ride a lot.
On a bike tour of more than a few days, you need to think about sustaining yourself and your energy. Plan to eat what you like to eat, just more of it. Junk food excluded, in my experience, mainly because the high fat/greasy stuff is hard to digest while rolling up the road. Although french fries with lots of salt can sometimes be great. Breads and potatoes are OK, cheese is a good lunch with some fruit and maybe a hard sausage, but for a long ride, start looking for more fruits and vegetables. They seem surprisingly scarce in rural grocery stores and cafes.
Make sure you keep the fluids flowing, as well. I think some people have been surprised at the steady $5/day Gatorade cost for extended touring. (I prefer orange juice and V8, myself...) And if you start feeling utterly wiped out, you probably need some salt -- pills, caplets, or sprinkled generously on a baked potato.
On the maximum calorie front, one of my daughter's classmates hiked the AT a few years back. Half way through he was so hungry that he stopped in a grocery store and ate a stick of margarine before he got to the checkout. (Collective "eeuw" from all his listeners.) He sort of shrugs when he tells the tale and says, "I was hungry."