Endurance is performing your duty under stressful conditions, for long periods of time. You can't think about quitting when someone is depending on you. The race is not always won by the swiftest, it is won by the one who endures until the end.

An unfit body or an undisciplined mind will never make it. Keep yourself fit, both mentally and physically. Learn to stand punishment by undertaking hard physical tasks. When you are tired, force yourself to study and think. Challenge yourself to go to the next level.