Definitely watch intake on that stuff, though. The guy who runs Gym Jones (hilarious name, and the gym that trained the actors in the movie 300) made some crack about watching people at most gyms, burning a whopping 300 calories on the stairmaster and then "replenishing" with a 500-calorie energy bar and drink.
You'll only need about 100 calories an hour to do pretty intense cardio. If you give your body too much, it won't turn to fat reserves to make up the extra. If you give it too little, though, you'll feel exhausted and have a lousy workout, because your body can't burn fat fast enough to use that as its sole energy source for exercise. "Fat burns in the flame of carbohydrates," as they say.
If you're only doing an hour, you honestly don't really *need* many carbohydrates during that exercise if you're well-hydrated and have eaten a meal within a few hours of starting.
Uh, so, a little more info, in case you're not already bored: your muscles have stores of this stuff called glycogen. Glycogen is a substance that can be burned in the muscle for quick energy. Think of it kind of like your car battery, where your digestive system & liver is like the car engine. Your "engine refills the battery", i.e. you synthesize and replace glycogen from blood sugar when your muscles use it up, and you use the glycogen like a car battery -- for a quick, short burst of energy that your "engine" can't totally provide.
At any random point in time, your muscles' glycogen stores are usually pretty full, and you only deplete them through intense exercise. And they'll probably last you the hour. And if they don't, you have enough residual blood sugar from your most recent meal to make up the difference.
I'd say, basically, experiment and see how few calories you can consume while doing your cardio, and then have a good meal with a sizable amount of protein and some good, whole-grain carbs after the workout.
You'll know when you're taking in too few carbs because when you run out of blood sugar, it's not subtle. You really just plain run out, it's like hitting a wall. But on just an hour of cardio, unless you are working yourself into a foaming lather, I bet plain water would be enough.