Well, here's some totally inconclusive evidence for fifteen miles:

We actually went on a barge on the New York State Barge Canal (formerly the Erie Canal) and were told that mules were changed every six hours. At fifteen miles per shift, that works out to 60 miles a day or six days to make the 360 mile trip from Albany to Buffalo - exactly how long it actually took.

That's why we've always sung fifteen miles in our school programs which allowed us to get the kids to do the math described above. We would tell them it was 15 miles in each 6 hour shift and ask them how many dayds it would take to do the 360 miles. In the average fifth grade class there would be at least one kid who could get the answer in his/her head.

Bev and Jerry