I think most people fall into two categories (there are always exceptions.) They either exercise regularly and if transportation miles can be worked into that exercise, then it's just a bonus. I know that if I didn't bike to and from work then I'd just come home and run. OR they have a little bit extra around the middle. The average american is something like 25 pounds overweight? How many miles is that?

I suspect we could power the entire single rider transportation system in the US from only body fat for something to the tune of a few months.