Why can I run longer on a treadmill than just plain running?

When you are running on the treadmill the "ground" or the belt you are running on is moving. When your foot strikes the moving belt on the treadmill you do not have to exert as much energy as you do when running on the road. When you run on the actual pavement you are forced to not only to move your legs but also to push off which in turn uses more energy than that of the treadmill.