Tennis star Caroline Wozniacki stunned the endurance world Sunday when she finished the New York City Marathon in 3 hours 26 minutes 33 seconds. It was her first marathon. She never did a training run longer than 13 miles, and she played in the WTA Tour Finals in Singapore the previous weekend. How fast would Woz be if she tried?

Pretty darn fast. Michael Joyner, a physician at the Mayo Clinic and veteran marathoner who specializes in time extrapolations, posited that at 5 feet 10, Wozniacki would most likely be better suited...