It is difficult to calculate the exact number of calories a person is burning while performing an activity, without using sophisticated equipment.
The amount of calories burned walking will vary according to the distance, speed and the style of walking.
In General, walking at slow speeds compared to walking at normal speed, will burn more calories due to you stopping and taking each step slowly, and the momentum not helping you to carry your body forward.
Walking at very highs speeds, you will burn extra calories due to the extra muscle movements with arm motion.
Running will burn much more calories per mile due to the up and down motion, lifting your weight off the ground and moving it forward at the same time.
The average person (body weight: 170-180 pounds for men and 150-160 pounds for women) burns roughly four calories per minute while walking at a pace around three miles per hour.