People often ask me the following question:
I usually do some type of aerobic exercise for 10-20 minutes, three or four times a week, but I hardly notice any difference in my body as a result. Why is cardio not working for me?
I used to wonder the same thing myself and then one day, in my own search for the truth about health and fitness, I discovered an interesting fact in one of my exercise physiology textbooks. This little nugget of truth has become one of the core fundamentals for my own personal beliefs about exercise and fitness.
Like most people, I used to spend fifteen or twenty minutes performing some type of cardiovascular exercise and assume that I was burning fat the entire time. Upon further investigation, I realized, however, that the process of burning fat as a fuel is a little more complicated than I once believed.
At the beginning or warm-up stage of an aerobic exercise session, the primary source of energy that your body is burning as fuel comes from carbohydrates, which are stored as glycogen in your muscles. For the next twenty minutes or so as you continue exercising, your glycogen stores supply only about half of your energy requirements, with the other half being provided by the breakdown of fat. 
Now, here is the really interesting part: The longer you exercise past this point, the more your glycogen stores are reduced, and a higher percentage of energy is supplied by the metabolism of fat.
To put it simply: In the warm up phase of aerobic exercise, you use mostly glycogen from carbohydrates. The longer you continue exercising, the more you will burn fat as a source of fuel. Only after about twenty minutes does your body start to use fat as its primary energy source.
The bottom line is that to begin truly burning from your fat stores, you may need to participate in cardiovascular activities and exercises a little longer than you are accustomed to. I have found that adding some variety to your usual aerobic exercise routine and including outdoor activities that you truly enjoy make it easier to obtain the proper amount of cardio that you need for a healthy balance in your daily life.
The wonderful thing about lengthier aerobic sessions is that they increase your metabolism long after you stop sweating, in some cases for up to six or seven hours. In addition to the fat burning effects, regular physical activity enhances mental health, greatly reduces the risk of dying from coronary heart disease, and also decreases the risk of developing diabetes, hypertension, and colon cancer, according to The Surgeon General’s Report on Physical Activity and Health.
If more than twenty minutes seems like a lot to you, concentrate on simply taking it one step at time. The key word is “gradually”. It is crucial to listen carefully to your body and steadily build a healthy momentum that is right for you and in line with your goals and physical condition. Remember, play it safe, stay active and, above all, have fun!
 Reference: “Exercise Physiology (Energy, Nutrition, and Human Performance)” McArdle, Katch, & Katch
 Surgeon General’s Report on Physical Activity & Health