Cardiovascular fitness is the heart and lungs' ability to supply oxygen-rich blood to the working muscles to use oxygen to produce energy.
Cardio increases the muscles' ability to use fat as a source of energy. It increases the size and number of cellular units (mitochondria) that produce energy aerobically. In other words, cardio doesn't burn fat or muscles like magic. Cardio "teaches" our body to use fat as a source of energy instead of "keeping it for just in case."
There are many other benefits of cardio. And fat loss is a widespread reason why people start walking running or getting on that treadmill. But many people still do not see any results after spending hours on the cardio machines.
As I said earlier, just by starting walking in your usual way of walking, you will not magically lose your fat. More likely, the energy that you will use is going to come from the calories that you ate today. To get any benefits from cardio, including fat burn, you need to exercise at 55-85 percent of your predicted maximum heart rate.
How to calculate your HRmax:
HRmax = 220 - your age.
Your exercise heart rate calculation:
Exercise heart rate range: (HRmax x 0.55) to (HRmx x 0.85)
The lower range is recommended for those how have been relatively sedentary. For those who are physically active, getting to the higher end of this range is recommended.
Stay healthy and use the time that you spent on cardio effectively!