Replace the question mark with a number

A guy is hired to paint lines on a little country road, so the boss gives him a big can of paint, a brush and sends him out... At the end of the day, when he comes to get paid, he tells the boss he got two miles done. The boss is pretty impressed. At the end of the second day, the painter reports that he did half a mile. The boss is a little surprised at the drop, but thinks maybe the first-day enthusiasm just wore off. At the end of the third day, the painter reports that he did 400 yards. The boss says, "That's quite a difference from the first day." The painter replies, "Yeah, well it's a lot longer walk back to the paint can now."