My triathlete friend Chris wrote a nice post a while back with some nutrition tips. It covers a lot of the same topics that I've been meaning to write about, so I'm going to use his post as a starting point.
Carbohydrates and athletes
Philosophically, my nutrition is very close to what Chris advocates - I eat one way for my normal diet, and eat differently around my workouts. As Chris notes, simple carbs are fine during exercise, but should be limited other times. The difference is because of the difference in the body's needs during the two periods, and the explanation is going to be long and have a few sidetrips, but I'll get there in the end.
Basically, your body has mechanisms intended to regulate your blood sugar so that it stays in certain ranges. Your brain, muscles, and other systems are constantly pulling carbohydrate out of your blood, and your digestive system is providing carbs back into the blood. Since mammals don't necessarily eat all the time, there are a couple of systems to smooth things out.
First of all, your liver stores a fair amount of glycogen, and it will release it to the blood as needed. It will also make you hungry. If you are exercising hard, however, you will get appetite suppression, and eventually, you will run out of liver glycogen. At that point, your body goes into a survival mode - it can synthesize enough glycogen to keep your brain going, but not support exercise at the same time.
This is the dreaded "bonk", and the confusion that you get as part of a bonk is because you don't have enough sugar in your brain. The amount of time it takes to bonk depends on how hard you're exercising (higher intensity requires more carbs), your level of fitness (high trained individuals burn fewer carbs at a given intensity), and how full you muscle and liver glycogen tanks are. So, some people can ride 3 hours without bonking, and others might sometimes bonk after 75 minutes. Be especially observant with kids, as they don't tend to eat as well or as often - my daughter bonked (or came close to it) on a bike ride last summer about 15 miles in because she hadn't eaten much recently. I always carry a couple of gels in my seat pack for those situations, and that made her happier quickly (though not happy, as it takes days to recover from a bonk).
So, anyway, that's why having a supply of carbs during exercise is a good idea, but as Chris notes, you don't need much - perhaps 150-250 cal per hour.
If there is excess blood sugar, it will go to muscle and liver glycogen. If those are full, the liver will convert them to fat and save them for a rainy day. That mechanism has served mammals pretty well historically, but it evolved for the typical mammalian diet, and a situation where food is scarce. It has a few problems with refined carbs.
Or, not really with the refined carbs, but with the stuff that has been refined out. You can eat foods that are high and sugar - such as fruit - but the absorption will be slowed down by the fiber in the food. Similarly, if your meal is a mixture of carbs, protein, and fat, the protein and fat will slow down the absorption of the carbs, and you will get a slow trickle of nutritents, which will keep you satisfied for a longer period of time.
If you eat the refined stuff - sugar, white flour, white rice - you blood sugar goes up pretty fast, and your body will likely have to store some of it in fat, and your blood sugar will go back down. So, that's why the whole foods are better from a carb perspective - they keep you full longer. Not to mention their other health benefits.
During exercise, things are different - your liver and muscle glycogen aren't full, and the small amounts that you should eat during exercise will go to keep those sources full.
It's important to get protein and carbs very soon after exercise, to refill the liver and muscle glycogen stores and start any needed repair. If you don't, your body will work to refill your glycogen stores by converting protein to glycogen. It gets this protein from your muscles, which would be bad. I have much less muscle soreness with carbs/protein drinks during and after exercise.
I've had great results with Endurox, and there are other recovery drinks out there. Low-fat chocolate milk is pretty good if you tolerate the lactose well.
The other huge advantage of a recovery drink is that it moderates your blood sugar, and you don't get super-hungry after the workout, and then overeat.
Finally, you need to think about maintaining your sodium stores. If you are eating fairly well, you probably aren't taking in a lot of sodium, and you can easily burn through all of that sodium after a few hours of continuous exercise. Your sports drink may not provide enough sodium, so you may need to consider supplementation