How many calories would you burn walking a football field?

I work from a really simple formula that works for running or walking:

one calorie per kilogram of bodyweight per km or "calories = kg per km"

I know it is not exact but is close enough to work out dietary requirements MOST of the time. Example:

90kg person walks 10 km burns 900 Calories. One still needs to add in the basal metabolic rate of "calories = kg per hour". So that in the above example, in one day a 90kg person runs (or walks) 10km their total calorie for the day equals:

90 kg x 24hours = 2160 PLUS 90 kg x 10km = 900 =>3060 Calories.