I don’t have an answer, but someone else here does, Louis Chew answers on Quora.

This is David Goggins. He’s the toughest man alive.

There’s no doubt about it. Goggins is the only member of the US Armed Forces to complete SEAL training, US Army Ranger School, and Air Force Tactical Air Controller training.

Any of those accomplishments alone would have been impressive, but that’s not all.

He’s also the current Guinness record holder for the most number of pull-ups done in 24 hours. Alongside that record are multiple first-place finishes at the most brutal ultra-endurance events, which attracts the toughest competitors from around the world.

How does someone consistently push himself to his physical and mental limits all the time?

Let’s find out.

http://drifternews..com/2018/02/digest-how-do-you-develop-mental.html?m=1