There is good news: your body is constantly at work; taking in and expelling oxygen, growing hair, constantly circulating blood through your system, replacing skin, digesting food, and doing the myriad other things that the human body must do to maintain itself, even in sleep, when the body and mind are busy repairing themselves. In fact, for most people, about 80% of overall caloric burn is simply the result of passive body maintenance. The most energy intensive thing your body does is thinking; the brain is the organ that consumes the most energy.
The calories you burn in exercise are at the margin; If you are ingesting 2500 calories a day through food and drink, and burning 2,000 through normal passive activity, you might burn another 300 through your routine daily activity; you brush your teeth, walk to the kitchen, carry some groceries to the car, play with your cat; every activity has some caloric cost. In this case, without any exercise, you would be burning a total of 2300 calories a day, while taking in 2500, thus you have a calorie surplus of 200 calories a day; these will be stored as fat. If you simply added a half hour of moderate exercise, you move from a calorie surplus to a calorie deficit even while making no changes in your diet. You would lose weight. Think of your checking account, but in the opposite direction: if you put more money in than you take out, your balance will grow. If you take more out than you put in, you've got problems. If you put more calories in your body than you burn, you will gain weight.
We think exercise is great; however, it's really not the easiest way to lose weight. To go back to our basic premise that burning calories is a whole lot more difficult than consuming them, the easiest way to lose weight is to eat less. Returning to the example above, if your normal intake has been 2500 calories a day, and you were burning 2300 through normal body maintenance plus your routine activities, you could just drop desert from your diet, decrease your intake by, say, 300 calories, and move to a calorie deficit. Which is easier: Abstaining from desert, or doing a half hour's exercise? For most people, the answer is clearly the former. Exercise is hard work; abstaining just requires a little discipline. We don't endorse a passive approach to weight loss, as there is a huge difference between losing weight and being fit, but if your objective is simply to lose weight, just eating less is the simplest approach.
While obesity is a huge health issue with wide ranging repercussions, fitness is much broader in scope; a truly fit person will be at their appropriate body weight (not necessary thin), but will also be strong, able to sustain activity for extended periods, reasonably flexible, free from daily pain or aches, and well rested. All of this will be reflected in their mental outlook, which should be positive and exude a sense of efficacy; a healthy and fit person will believe they have control over outcomes. By exercising control over their bodies, they gain an overall sense of control and mastery.
We've all seen pictures of stick thin teenage models; they may be beautiful in some narrow esthetic sense, but they look weak and unhealthy. The pictures in the margin show the Attitude Media vision of fitness and health.