The answer is really quite simple, according to this researcher. British units of measurement ("which now only the Americans use, the British having gone metric like everyone else") include a fundamental unit called the rod (aka the pole and the perch), which in the 16th century was defined as the total length of the left feet of the first 16 men to leave church on Sunday morning.

There are four rods to a chain, 10 chains to a furlong, and 8 furlongs to a mile. Since each rod is 16.5 feet in length (the average 16th century male churchgoer's left foot being a little over 12 inches in length), it all adds up. And you guessed it - there are five toes to a foot!

Image credit: The Dynamics of Progress: Time, Method, and Measure. Samuel L. Macey (University of Georgia Press, 1989).