In Jesus' day, a Roman solider could tap you on the shoulder with the flat of his spear and force you to carry his pack for one mile. You could refuse if you wanted him to adjust his spear and ram you with it. Jesus understood the hatred and fear most people had of these occupiers of their land so He tells them about the second mile.
To go the second mile is to do the unexpected, to go to the uncomfortable part of life; yet Jesus knew that those of us who do receive a double blessing, one of helping and one of being helped.
Where is that area today you're comfortable barely doing one mile? What do you and God stand to gain if you go two?