How you burn fat: the science
Using energy comes from breaking down carbohydrates, or glycogen, into simple glucose molecules -- this process is called glycolgenolysis. Your body breaks down fats into glycerol and fatty acids in the process of lipolysis. The fatty acids can then be broken down directly to get energy, or can be used to make glucose through a multi-step process called gluconeogenesis. In gluconeogenesis, amino acids can also be used to make glucose.
In the fat cell, other types of lipases (enzymes) work to break down fats into fatty acids and glycerol. These lipases are activated by various hormones, such as glucagon, epinephrine and growth hormone. The resulting glycerol and fatty acids are released into the blood, and travel to the liver through the bloodstream. Once in the liver, the glycerol and fatty acids can be either further broken down or used to make glucose. Once the glucose has travelled through the blood stream and through the body they remove the lactic acid which turns in to carbon dioxide and out through the airways, so ideally the better you are at breathing and passing oxygen through the body, the better your body will be at burning fat!
Low Intensity Cardio
Walking, swimming, running and jogging fall in the category of low intensity cardio. When trying to keep a slower pace your body focuses on your aerobic system to produce energy from fat stores and your oxygen. We don’t really need large stores to explode like phosphate energy does, so this means that it is localised to burning fat. Just because it burns fat doesn’t actually mean that it is the better option for losing weight, but mainly an optimiser for increasing your cardio abilities, so in turn your body can use its energy better. Low intensity cardio is a good option for getting fit, rather than using it just to burn calories. When doing any long periods of exercise your body goes in to a stressed state that releases a hormone called cortisol. Cortisol plays a part in catabolism (the body eating it's own muscle) and is a reason why some people don’t see muscle gains and can stereotypically explain why you don’t see too many bulky runners or joggers. The ideal length of time for a low intensity cardio session is between 30 and 45 minutes.