First 5 minutes
As soon as you start walking, your body starts releasing lubricating fluids for your joints and energy for your cells to use. Even your heart rate increases slightly. You start burning anything in the range of less than 5 calories per minute of walk.
Another 5 minutes
Once you have walked for more than 5 minutes, you start burning around 6 calories per minute as you increase your pace. Your heart rate also increases and stays between 100 to 140 bpm. At this point your body also start feeling slight increase in blood pressure, which is countered by your body with the help of chemicals that expand the blood vessels.
Another 10 minutes
A 20 minutes walk will get your temperature up and the blood vessels near the skin would expand to release the heat; which in turns results in sweating. Now as your pace increases you'll be burning up to 7 calories a minute. You may also be breathing harder.
From 20 to 45 minutes
Once you have walked for 20 minutes your brain starts releasing chemicals such as endorphins that help you feel better. As you burn more fat during the next 25 minutes, your insulin levels drop. Since insulin helps store fat, drop in insulin levels is good for people with diabetes or excess weight.
45 to 60 minutes
Your muscles start feeling fatigued as more fat has been burned out. You will keep burning more calories for another hour or so. The heart rate and breathing will gradually return to normal as you slow back down and walk slowly due to the fatigue.