Metbolism and weight gain/loss are very complicated issues. When you train hard (70% to 80% of maximum heart rate), the body cannot convert fat to energy fast enough, so you use up your glucose and glycogen stores until you bonk. As soon as you stop and eat, you replace the glucose and glycogen stores, and there is little net change in weight.
When you exercise at a lighter level (50% to 60% of maximum heart rate), the body can burn fat instead of depleting glycogen stores, so you lose weight faster (assuming you have excess fat), and you can exercise longer.
Another factor is that not all fat is the same. Gut fat is bad, and butt fat is good (or at least not bad) as far as overall health is concerned. Gut fat (i.e., peritoneal fat) is more metabolically active than butt fat (which actually incompasses all subcutaneous fat). When fat is stored, the gut fat accumulates first, and when fat is used for energy, it is the gut fat that is used. Typically, the first ten pounds lost is gut fat. Once the gut fat is gone, it takes the body time to redistribute subcutaneous fat to gut fat, and the body cannot utilize subcutaneous fat quickly enough to use it as an energy source during exercise. Periods of inactivity between exercise allows for the remaining fat to be redistributed so that when exercise resumes, the gut fat can be burned.
That also explains, in part, why weight loss tends to go in in steps. You lose 10 pounds fairly easily, and then you hit a plateau. After a while, you go through another period of rapid weight loss, and then hit another plateau.
The good news is that when you are losing weight, no matter how overweight you are, the biggest improvement in health comes from losing the first ten pounds. The bad news is that when you are gaining weight, regardless of how close to your ideal weight you might be, the biggest hit to your overall health comes from the first ten pounds gained.
