The idea that you will lose weight simply because you start exercising is a common one, but the truth is that the results of this type of intervention varies greatly. Burning 7000 calories will, on average, lead to losing 1 kg of fat, but this is in no way applicable to everyone. Different people respond differently to physical activity and some may lose 0.1 kg while others lose 2.3 kg for the same amount of exercise. Some people even gain weight.
The fact is that you generally cannot out-exercise a bad diet. Very few people (elite athletes, marathon runner) lose weight through exercise alone. So if your goal is to lose weight, you should first and foremost adjust your diet, either by choosing healthier foods, or by counting calories to make sure you are in a caloric deficit. This combined with exercise is the best way to lose weight. However, also remember that exercise has many other health benefits even when it doesn’t lead to weight loss!
Source 1: Seabra AC. Effects of a 5-month football program on perceived psychological status and body composition of overweight boys. Scand J Med Sci Sports. 2014 Aug;24 Suppl 1:10-6.
Source 2: Cook CM. Physical activity and weight control: conflicting findings. Curr Opin Clin Nutr Metab Care. 2011 Sep;14(5):419-24.
Source 3: Wilks DC. Objectively measured physical activity and fat mass in children: a bias-adjusted meta-analysis of prospective studies. PLoS One. 2011 Feb 23;6(2):e17205.
Source 4: Metcalf BS. Fatness leads to inactivity, but inactivity does not lead to fatness: a longitudinal study in children (EarlyBird 45). Arch Dis Child. 2011 Oct;96(10):942-7.
Source 5: Luke A. Energy expenditure does not predict weight change in either Nigerian or African American women. Am J Clin Nutr. 2009 Jan;89(1):1