When Jesus became an adult, the law still stood. And Jesus could have easily talked about how unfair this law was. He could have told the Jews to no longer carry these burdens for the Romans. He could have told the Romans that the Jews would no longer be human donkeys. But instead, Jesus said something very interesting.
Twain means two.
Jesus was telling them to carry the burden two miles instead of one. By doing this, they would demonstrate their willingness to get along. They would demonstrate their desire to serve all men. And by doing this, they were showing that they were willing to go the extra mile. The next time you think a rule is not fair, the next time you do not like a rule your parents make, the next time you think you are being treated wrongly, think about Jesus talking about going two miles instead of one.