I understand when physiologists study runners and running, they put the runner on a treadmill and make him breathe into a tube that connects to devices which monitor carbon dioxide. From this it's determined how much fat is being burned during the exercise. That's where the "fat burning zone" idea comes from.

But you're right, that at a given distance the same number of calories are burned regardless of pace. There was a study using two runners with about the same size and ability. One did a 4-hour marathon on a treadmill. The other runner did a 3-hour marathon on a treadmill. They each burned exactly the same number of calories but the 4-hour runner burned a higher percentage of calories from fat.

The article didn't go into it, but I guess the 3-hour runner stimulated a greater degree of Growth Hormone post-run which lead to accelerate fat loss post-run. Just a guess.