I lead a very active lifestyle. That means while I'm mountain biking, cycling, or running, I need to stay well hydrated and nourished. On weekends, I might spend three or four hours mountain biking. In the winter, I still run and mountain bike outside, the dry air in New England means it's even more important to stay well hydrated when exercising outdoors.
Let's take mountain biking for example, I can burn thousands of calories in one ride. Without regular refueling, I'd bonk, and that'd be bad. Here's another good article on the science behind bonking. I have found that during these extended periods of exercise, my magic number is around 45 minutes. This means I need easily transportable nutrition and I need to eat around every 45 minutes. If I delay eating, or skip a snack all together, I'll lose energy fast.
So what do I eat? Gu energy gels and Clif bars. I've tried many other brands of energy gels and bars and found these to be my favorite. My favorite Gu energy gels are the Vanilla Bean, Espresso Love, Chocolate Outrage. My favorite Clif bars are the Peanut Toffee Buzz and Cool Mint Chocolate. I find the flavors of the Clif Shots energy gel and PowerBar energy gels to be unpalatable. I also have a CamelBak hydration pack that I fill with 70 ounces of water and drink from frequently.
One last note on energy bars, in a 2004 Consumer Reports study, it is reported that Americans spend $1.4 billion per year on energy bars, also marketed as sports bars, meal replacement bars, or nutrition bars. Brands such as Powerbar, Clif, Clif Luna, Carb Solutions, Atkins Advantage and Ultra-Slim Fast. There are others that are much more athlete specific such as ProMax, Pure Protein, and Met-Rx. Find what you like, and more importantly what your stomach can handle during exercise, and then look volume discounts from retailers like REI.