Allow me to paint a picture for you...

You're running your first Ironman.  You've completed the 2.4-mile swim.  You've survived a 112-mile bike ride.  You've run for 26 miles.  You've done all of these things in the blistering Summer sun.  It's unnatural, humans shouldn't be doing this to themselves.  You've a quarter mile left, you can see the finish line that you've labored for hours to reach.  You push yourself further still, harder, you have no energy, you run on pure will.  You reach the finish line, arms raised in victory, you feel ready to collapse.  Tears, Exhaustion.  Then at once all you can hear is a very clear voice that states: "You've gotta keep going."