Our ancestors were concerned with survival, not competing, which is why we are programmed to back off when we become excessively fatigued or in pain during a race. Our brain acts as a central governor with a safety circuit to protect us from any long-term harm. However, we can extend the boundary of our circuitry over time and override the switch in the short-term to achieve a breakthrough race. The notion is repeatedly illustrated by racers who say they are “completely dead” and unable to go faster during the late stages of a race but are able to override their circuitry to muster a finishing kick when the finish line is in sight. By understanding how the brain works and why we are receiving certain signals throughout a run or race, you can develop the ability to quash your instincts. Adapting to resist mental fatigue as much as physical fatigue is crucial. Nearly everyone has similar pain thresholds, but upping pain tolerance is the key to achieving the next level, as described in the book "Endure" by Alex Hutchinson. @Jingxuan