Heart Rate and Fat burn (heart burn and fat rate?)

Although I haven't been to the gym in months, I was always mystified by the heart rate scales on the treadmills and elliptical machines. For some reason if your heart rate stays lower you are in "Fat burn" territory. However, when your heart rate gets higher your workout is considered "Cardio."

WTF?

I thought the whole point of doing cardio workouts was to burn fat?

I read this fantastic book, Marathoning for Mortals, which I highly recommend to anyone even remotely interested in running. Here's a section from their book that breaks it down:

"You run all of your tempo workouts just below your anaerobic threshold, that is, the level at which your body begins to burn the short supply of glycogen or sugar in your system at a higher rate. Your muscles can burn one of two types of fuel–fat or glycogen (stored carbohydrates). At lower intensities–such as walking–your muscles prefer to burn mostly fat for fuel. However the higher your intensity, the more glycogen your muscles burn.

"You can go for days on fat stores but only a few hours on glycogen. The body is always using a ratio of both, but for the purpose of a successful and comfortable race, you want to burn a higher rate of fat.

"Your tempo workouts will allow you to raise your threshold. Ultimately, your body will burn a higher percentage of fat for a longer period of time at a higher intensity. This not only helps you lose weight; it also bolsters your endurance. The more your body burns fat for fuel, the more it conserves glycogen and the longer you can run without feeling tired."

Hey, I'm a fan of burning fat, So I think I'll keep up the tempo runs!

I mean, it makes sense that you can build endurance by increasing your threshold. And really I've seen evidence in my own running career.

Presently I am reading The Complete Book of Running for Women, where I discovered that women store the same amount of glycogen whether we eat a diet containing 60 or 75% carbohydrates, but we burn more fat than carbohydrates or protein compared to men. Which means our glycogen store will last longer.

So.... if I keep up the tempo work and maintain a healthy amount of carbs and fat in my diet I will be able to run forever...

Or cover the same amount of distance in less time...

I prefer the latter. Even though I totally agree with C.S. Lewis when he said, "If one could run without getting tired I don't think one would often want to do anything else," let's face it, comrades, the best part of distance running is when you're finished.