If I take 2 years or 20 minutes to walk a mile, I hardly think the number of calories burned is going to be the same. (Just pointing out the obvious for no good reason.)
Why is that obvious? It is not right. If it takes you two years or 20 minutes, you will burn about 100 calories in walking a mile. If it takes you two years, that will be an inconsequential amount considering how many calories you otherwise expend during that period. Even if it takes you 20 minutes, that would be an inconsequential amount unless you did that every day. In two years, walking a mile every day will cost you 10,400 calories. Since there are approx 3500 calories in a pound, in two years you would lose about 3 pounds, all other things being equal. Not much, so you may want to increase that to 4 or 5 miles, losing 12-15 pounds in two years.
If you walk at a pace of 1/10 of mile per day, you will burn ~20,000 calories moving a mile. If you don’t move at all, you will burn an infinite amout of calories before moving a mile. 
Brian
(sorry, couldn’t resist)