Suffering is a big part of running. The more suffering you can tolerate in races, the better you will perform. Some runners can tolerate more suffering than others, but all runners can increase their capacity to push through the discomfort of fatigue. Proven ways of doing so include regularly performing workouts that expose you to judicious doses of fatigue, aiming to beat your own best times in certain benchmark workouts, competing against other individuals in races and workouts, consciously rating your effort in races and key workouts, and getting motivation wherever you can.
— Matt Fitzgerald for Active.com