Running

To my mind when you reach the half way point in a marathon you're about 1/3 of the way in terms of physical exertion and pain.

Sounds like a nice summation.

I find that even in amazing condition the last 6 miles are shite. So I simply dont do them any more. A half marathon is as much a test of endurance, as most sane people need. Plus you can actually plan and adjust pace. As apposed to simply grtting your teeth and trying not to be crying* too hard when you cross the line.

(*mirrored running shades FTW)