The phrase "Go the extra mile" has its root in the New Testament. It stems from a Roman law allowing a Roman soldier to compel a subject to carry his gear for one Roman mile.
Of course, it's since been thrown into the abyss of pop self-help nonsense. The fact that it advises physical, unrewarded work is all but forgotten.
Whoever forces you to go one mile, go with him two. - Matthew 5:41 (New International Version).