Exercise is only 15-25% of dieting.
Many dieters mistakenly think that exercise is key to weight loss. Weight loss is 75-85% what you eat so you must eat healthy to keep the weight off and lose weight. If you do not watch what you eat, you will gain weight. I am not saying that exercise is not important. It absolutely is an important part of dieting. You must control what comes in to be successful at weight loss.
If you exercise for 30-40 minutes a day, you will burn 122 calories walking or 250 calories running. Sure this can make a difference if you do it 3-5 times a week for a year, but not near the difference cutting 500 calories out of your diet will. For cardiovascular health, exercise and healthy fats are essential, but for weight loss, cutting calories is needed.