Genetics shape us in many ways including our potential to excel in sports. Training, diet, and other factors play a large role in developing our potential, but our genes may also limit performance.
Genetics have a large influence over strength, muscle size and lung capacity, flexibility, and, to some extent, endurance.
One major limitation for endurance athletes is cardiac capacity, or the heart's ability to deliver enough oxygen (via the bloodstream) to the working skeletal muscles. This, too, is largely determined by genetics.
The other limitation for endurance athletes is in the muscles ability to effectively use the oxygen and create ATP (adenosine triphosphate), the fuel that allows muscular contraction and movement. The efficiency of this process is measured by something called maximum volume of oxygen.