Teach yourself to overcome mental factors in your training. The mind and body each want to quit at various times. Part of conditioning is learning to work through the points where you want to stop--not where you feel an injury or pain, but those times where you are fatigued, out of breath or feeling the emptiness in the pit of your stomach, as well as those times where your mind becomes overwhelmed with the effort, bored or frustrated.
Learn to work through the urge to stop by pushing yourself to that point on smaller courses and convincing yourself to continue to move, even for a short distance. Try for one more step, then 10, 20 and beyond.