No pain... no fun? For tower runners, running up skyscrapers is a painful yet challenging experience they see as the ultimate test

Running up stairs in dank, dusy stairwells -- with no views, no spectators and no way to know how you're faring -- may not seem like much fun. But to the growing number of tower runners on the global tower-running circuit (yes, there is one), it's the ultimate challenge. My story on tower runners and the tower running phenomenon for ESPN.com's Endurance Sports section: http://espn.go.com/sports/endurance/story/_/id/9513011/endurance-sports-tower-running-gaining-steam-globe