To run a freaking marathon.
To me it seems like the ultimate test of your in shape-ness. Can you imagine? What an accomplishment. It also wouldn't hurt having the satisfaction that I could say "WHAT NOW?!" to all of my elementary-high school foes who made fun of me for not being the most athletic or coordinated when it game to sports/gym class. If I should ever cross paths with them again, hopefully it will be after I ran a marathon. But who knows how many of them have already run one. Oh well. That's beside the point.