Final answer:
When runners race for about 20 minutes, their bodies obtain energy through aerobic metabolism in the muscles.
Step-by-step explanation:
When runners race for about 20 minutes, their bodies obtain energy through aerobic metabolism in the muscles. Initially, the muscle glycogen is broken down into glucose, which undergoes glycolysis producing pyruvate. The pyruvate then reacts with oxygen and enters the Krebs cycle to produce carbon dioxide, water, and energy. The glucose can also come from the liver, which releases it into the bloodstream.