How many calories does casually walking a mile burn for a 183cm/6ft man, weighing 180 pounds?
Walking 1 mile for someone weighing 180 lbs will burn 100 calories.
You can see from this calculator that walking for 20 mins at 3mph (3mph for 20 mins is 1 mile) burns 100 calories.
You can also see, using the same calculator that walking for 15 mins at 4mph (4mph for 15 mins is 1 mile) burns 107 calories.
So speed does make a minor difference, but in terms of weight loss 7 calories per mile are negligible.
So this is nice and easy to remember. A 180 lb person will burn 100 calories per mile walked.