Jim posted a comment on my century ride, talking about what I ate. He totalled up what I ate during that time period, and remarked on how small it was. I agree, though I get a number that's even smaller than the one he got:
330 calories in Clif Bars
520 calories in Accelerade
250 calories bagel
55 calories Newton
250 calories pretzels
That takes us onto the calories that I expended. The only good way to measure this is with something like a PowerTap, which is a wheel hub that measures how much force you're putting into your back wheel. Integrate that over time, and you get power, and you can make a good estimate of how much energy you expended.
Without that - and I don't think I'm likely to spend the money for a PowerTap or other power-measuring system, at least in the near future - you're stuck with estimates, which suck. The number of calories you expend vary depending on the speed you're riding, whether you're riding in a group, whether it's hilly, etc. I'm going to choose 500 cal/hour as a ballpark estimate. That means that for 5.5 hours, I spent:
5.5 * 500 = 2750 expended.
That doesn't seem so different from what I ingested, though I didn't eat my regular meals during that time (I ate less for breakfast and didn't eat lunch), which adds something like 900 calories.
2750 + 900 = 3650 expended.
for a net of:
3650 - 1400 = 2250.
So, where did the extra come from? It came from my fat stores. I've been doing some reading on this - Chris Carmichael's "Food for Fitness" (very good, by the way) and a few research studies - and it turns out that fat transport can supply a lot of calories over a long period, under two conditions.
The first is that the system is trained. The second is that there's sufficient glucose available to keep things going. To simplify, you need a constant supply of glucose to keep your metabolism going and your brain happy - if you get low on glucose, your brain will hoard it and you performance will tank, with the dreaded "Bonk".
Making another assumption - that I was getting approximately 80% of my energy from fat and 20% from carbohydrate, you'd expect that of the 3650 calories, 2920 came from fat and 730 from carbohydrates (okay, there's some in there from protein, but not a large proportion).
With the exception of the Clif bars, most of what I ate is at least 75% carbohydrate, so there's plenty of carbs to keep me going, assumming that it can get into the bloodstream quickly enough. That's where the Accelerade helps.
At this point, I should note that I'm not an expert at these things *and* that I've made a ton of simplifications along the way. For example, the brain uses a fair amount of carbohydrate, but that use can be supplied by conversion from fat to carbohydrate, so the carbs that I took in didn't need to support that part of my base metabolism, and were therefore free to be used towards the exercise.