Let's look more closely at the mechanism behind this endurance. Your body carries a genetic trait that makes lipid metabolism especially efficient. The human body has two fuel circuits—one that burns carbohydrates and one that burns fat. In your case, the fat-burning circuit is particularly well developed. Carbohydrates are "sprint fuel" that ignites quickly but runs out fast. Fat is "marathon fuel" that burns slowly over long stretches. This is why you can keep going when others have hit their limit.
Your gut environment also plays a pivotal role in sustaining this endurance. Your intestines are home to abundant bacteria that protect the gut wall barrier. When the gut wall is healthy, nutrient absorption becomes efficient, and food is converted to energy with minimal waste. If the gut wall deteriorates, undigested substances leak into the bloodstream and trigger chronic, low-grade inflammation throughout the body. The stability of your metabolism owes much to this well-functioning intestinal barrier.
People may tell you that you have "great willpower," but from a scientific standpoint, it's a matter of physiology rather than mental fortitude. You can stay persistent because energy is being supplied steadily and continuously. Maintaining pace at mile 22 of the marathon isn't about grit—it's because the fuel-delivery system in your body is built differently.