Albert Barnes on Matthew 5:41

Whosoever shall compel thee to go a mile. The word translated shall compel, is of Persian origin. Post-offices were then unknown. In order that the royal commands might be delivered with safety and despatch in different parts of the empire, Cyrus stationed horsemen at proper intervals on all the great public highways. One of those delivered the message to another, and intelligence was thus rapidly and safely communicated. These heralds were permitted to compel any person, or to press any horse, boat, ship, or other vehicle that they might need, for the quick transmission of the king's commandments. It was to this custom that our Saviour refers. Rather, says he, than resist a public authority, requiring your attendance and aid for a certain distance, go peaceably twice the distance.

A mile. A Roman mile was a thousand paces.

Twain. Two.