Some Navy SEALs follow a saying that, when you feel that you're completely wiped out, you're actually only 40 percent done and have 60 percent in the tank.
It's an idea popularized by David Goggins, a SEAL who completed 14 races that were each over 100 miles long, and he did it most of it while on active duty with a potentially fatal heart defect that limited him to about 75 percent heart function.
Jesse Itzler, a billionaire who competes in some endurance events, met Goggins during a 100-mile race, one of Goggins' first. Itzler was running as part of a 6-man relay team. The SEAL was running the same race on his own at 260 pounds.
According to Itzler, Goggins' feet and kidneys were shot when they met on the course. But Goggins kept going and, after they finished race, Itzler asked Goggins to come live at his house for a month and teach his family about mental resilience.
Itzler later wrote a book about that month and related stories of how the SEAL pressed him forward. Goggins did things like teaching Itzler to do more pull-ups than he ever thought possible by making him do 100 more after he thought he was already exhausted from doing about 18.
Goggins taught Itzler to leave his comfort zone by telling him about the 40 percent rule, which basically says that the feeling that you're completely tapped out actually comes when you're only 40 percent done; you still have 60 percent left in the tank.
And that's what most of the world knows about Goggins, but his story is actually more amazing than he gets credit for.
He kept running and began competing in triathlons despite a hatred of running. He did it to raise money for the families of fallen special operators, and he raised over $200,000 in three years by running more than 14 races of over 100 miles.
Since beginning to run ultra-marathons in 2005, he has completed more than 50 of them. Clearly, the 40 percent rule works pretty well.
He competed in the Badwater 135 less than a year after running his first marathon and he placed fifth. He ran the race again a year later and finished third by running 135 miles in 25:49:40. That's about 11:30 per mile for 135 miles.
Oh, and he did most of this while still on active duty as a SEAL. And he did most of it with a potentially fatal heart defect that was surgically corrected in 2009. The defect limited him to approximately 75 percent heart function before he was treated.
Goggins hadn't known about the defect when he joined the Air Force and became a tactical air controller, or when he completed Army Ranger School, or when he completed Basic Underwater Demolition/SEAL training (BUD/S).
But he had the defect from birth and just kept pushing himself.
Goggins now completes speaking engagements where he focuses on pushing people out of their comfort zones and convincing them to stay there.