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A lot of the above differs from what I was taught in college as an exercise & wellness major, too, Zaf.
First of all, it's true that the body uses 'carbs' first, but not the ones you just ate. The body stores energy as glucogen, there are small reserves of it in the muscles, ready to be used if you should suddenly decide to sprint across the parking lot in the rain, for instance. There's lots more glucogen stored in the liver & (IIRC) other places in the body.
After about 20 minutes of activity, the body is likely to shift to 'fat-burning' mode. This figure is pretty approximate, depends a lot on how efficient your body is at producing energy (how fit you are).
So, in order to burn fat, you're better off exercising at a low level because you need to sustain your activity for at least 20 minutes before you begin to burn muscle. Hence the easier 'fat-burning' settings on the machines. If you want to achieve cardio fitness, you need to get your heart rate up to 60-80% of your VO2 max, which is much more intense. You would still be burning fat at that range, but you don't *have* to exercise that hard to burn fat.
I think you're right to look at calorie intake (you're also right that we usually mean Calories, with a capital C, which is a kilocalorie). That's the other side of the equation. I used to walk an hour on the treadmill & then figure I could 'afford' to eat more. Nuh uh! An hour on the treadmill and a tiny bag of peanut M&Ms cancel each other out. Or a bowl of soup. Never mind a sandwich at the deli! A plate of my favorite pasta dish = SIX HOURS on the treadmill. My fave chocolate milkshake -- a small one -- would take about 4 hours to burn off, walking hard.
Counting calories is a great way to get a better understanding of it all. So is the Weight Watchers point system. I'm using Fitday.com right now, cuz it's free.
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