An old school of thought on walking is that because it is a low intensity exercise (that is, your heart rate is not as high walking compared to running, which is higher intensity), you are in the "fat burning zone".
This old school theory also states that high intensity exercise burns glycogen (stored carbohydrates) only, and not fat.
While there is some overlap where both fat and glycogen are burned during exercise, during low intensity exercise more fat than glycogen is burned, and during high intensity exercise more glycogen than fat is burned.
The theory is you want to burn fat, not glycogen.
The main problem with this is once you have completed your low intensity "fat burning" exercise, you will no longer be burning the same amount of calories.
By the way, low intensity fat burning exercises also include walking to your mail box or even sitting down checking your email.
When you exercise at a higher intensity, your metabolism is raised so that you continue to burn more calories after the exercise.
If you do the right types of exercise, your metabolism will be raised for 24-48 hours afterwards and this is much preferable than going for a low intensity walk.
If you are obese or have a sedentary lifestyle then walking is a great introduction to exercise.
It is low impact, which means it is kind to the joints and lessens the risk of injury, and if you have no history of exercise, then walking briskly will give you improved cardiovascular health and likely some reduction in body fat.
With all forms of exercise, you must pay attention to the principle of adaptation and the principle of overload.
The principle of adaptation shows itself very quickly when you exercise.
Your body adapts very quickly as you become better at the activity.
If you start your exercise habit by walking briskly for 30 minutes a day, at first you might find it challenging.
As you keep doing this, you gradually find it easier and as you adapt your heart rate will not be as high, you will find yourself exerting less effort and because the body is so good at being efficient, it does not burn the same amount of calories as it used to.
This means you need to introduce the principle of overload.
You must overload your body to keep seeing improvement.
With the example of walking, this could mean walking faster or walking longer.
There is a limit to how fast you can walk so you will have to walk longer to adhere to the principle of overload.
Walking for 45 minutes is OK for a while..
Until your body adapts to this! Then you will need to walk for 1 hour or 2 hours...
or all day to get the same benefits you once got from walking 30 minutes! This is clearly not practical and this demonstrates why walking is not a great exercise choice for burning fat.
So what is the best exercise for burning fat? The best form of exercise is resistance training.
This means lifting weights.
You can exercise with weights for 30-45 minutes and never have to increase this time frame.
By applying the principle of overload, all you need to do is increase the intensity of your workout.
This could be by reducing your rest periods, increasing the number of repetitions, increasing the weights used or introducing more difficult exercises.
Not only can you manipulate the principle of overload easily with resistance training, but you will build more muscle, which will increase your metabolism to burn more fat, which is exactly what you want! After all that, walking is not a bad form of exercise.
It's a great way to see new cities, enjoy the bushland and improve cardiovascular health.
Just don't expect anything more than that.