Why is a mid-foot or forefoot strike more efficient than a heel strike?
If you are landing on your heels, then you are probably landing when your foot is out in front of your body. If your foot is out in front of your body when it lands then it is absorbing your forward momentum. As has been said by others, you are essentially putting on the breaks with every step. For a more detailed discussion, see Explosive Racing or Programmed to Run.