The number of calories used up by exercise is often less than expected, but despite this, it is much easier to lose weight, and also avoid gaining weight, if you exercise regularly.
Our bodies are made for movement. The people who we are descended from, devoted most of their time to physical activities. The normal weight regulation system functions better if we exercise. Overweight occurs when this system doesn't function correctly and you want to eat even when you should be satified. Physical activity helps the body to feel hunger and satisfaction in the right way. A combination of exercise and avoidance of fat and junk food, which distort our ability to feel satisfaction, is important. The effect of exercise on the intake of food was first discovered during research on rats. The rats were allowed to exercise at different times per day and it was found that when they exercised for one hour or more, their eating increased in accordance with their calorie requirements, but when they exercised for less than one hour their eating increased anyway instead of getting less. The body's natural weight regulation no longer functioned when they exercised for less than one hour. Practical experience indicates that exercise does make it easier also for us to keep to a certain weight. You will eat less if you exercise. (Plus, it is impossible to eat while you are exercising!)