Cool post on the Final Sprint about heart size and endurance:
What laboratory test is the best predictor of finishing times for a running race of 100 kilometers (62 miles), or more than twice the distance of a marathon)? A study from Yokohama, Japan suggests that it is an echocardiogram to measure the size of your left ventricular heart chamber (British Journal of Sports Medicine, Volume 40, 2006), which determines how much blood your heart can pump with each beat.