Exercise has nothing to do with weight. Weight is about food. Exercise is for fun, health, strength, speed, endurance, flexibility, sanity, and general awesomeness. It's not about calorie-burning because it doesn't burn enough of them to make much difference. Think about how hard you have to exercise and for how long to burn 700 calories. Then think about how easy it is to eat 700 calories. You can do it in a few seconds and 4 bites. Food is what matters. Eat with awareness and according to appetite and you can lose weight (or maintain your weight) with zero exercise, as many of us have learned when we break something and can't train for weeks. Separating exercise from weight loss was one of the most freeing breakthroughs I made on this journey.
I used to worry that if I didn't exercise, I wouldn't be able to eat as much, and that would be unbearably sad. The truth is that appetite changes along with activity level, so you can always eat to complete satisfaction. You imagine that if you eat any less, you'll feel deprived. What most of us can't imagine is being legitimately less hungry, being totally satisfied, or even too full on less food, especially if we're chronic overexercises. It's never happened, so we don't think it's true or possible, not for us. We're broken. Or, maybe we stop exercising for a day or a week and are still ravenous. We allow this to confirm our fears and go right back to hard exercise, never giving it long enough for appetite to settle down. If you've been training hard on too little food for awhile (years for most of us), it's not going to sort itself out immediately, but in a couple weeks of little or no exercise, you'll be a different person. There will be a casualness around food that's hard to imagine. I went most of my life cleaning my plate, licking the container, watching the clock until I could eat again, really just feeling greedy and obsessed because I was so hungry. Now that I'm eating adequately and properly recovering from my workouts, I'm casual, I share, I'm easily satisfied, and not thinking about food at all the vast majority of the time.
Another amazing observation is that my energy expenditure and food intake with and without exercise is very similar. If you've been exercising since the dawn of time, especially with cardio, it's no longer the calorie-torching extravaganza you imagine. Our bodies become super efficient at whatever we subject them to regularly. I remember a conversation I had with Alwyn Cosgrove about the futility of cardio for fat loss, specifically running and fat gain. He mentioned a study where people who added an hour of aerobic activity per day, six days per week for 52 weeks lost barely 3 pounds, and another one where men who logged the same weekly running mileage, even at over 40 miles per week, gained weight.
I think the adaptation to the activity (ie the metabolism slows down) occurs at an extremely fast rate. So
1) they lower metabolism quicker than they burn calories
2) the activity does nothing to maintain lean mass - in all actuality - it likely reduces lean mass (and lowers metabolism further) and
3) The adaptation to running - means that they don't burn as many calories doing it as they originally did.
My other more esoteric idea is the homeostasis one. When you weight train - you break down muscle -- and the body adapts by growing more muscle. When you undereat - your body adapts by lowering metabolism in the short term. When you overeat - your body speeds up metabolism for a short time When you drink too much water - you pee more - when you don't drink enough - you retain. The body adapts to any stimulus by doing the reverse.
So when you do an aerobic activity (that burns fat) maybe your body adapts by storing fat?
This was many years ago, but it squashed my cardio queen dreams. LOL Personal experience tells me there's something to it. Excessive cardio makes me hungry and tired. Hungry and tired people eat more and burn less. Ouch. Never mind.