This was a reference to a law by which the Roman could force a Jew to carry some goods for one mile and one mile only. Therefore, here Jesus meant that, if you just go one mile, there is nothing special, as that is the law. You must go further than that, by will and love. It might also have been an indication that the new covenant does not revolve on following the law, but in a new measure of love.
You can read it much better in this bookthis book, and the relevant quote below: