Retired Navy SEAL David Goggins has a saying: When your brain says that you're done, you're only 40 percent done.
It's something Jesse Itzler, an Atlanta Hawks owner and co-founder of Marquis Jet, reminds himself of after having had Goggins as his live-in personal trainer for a month.
Goggins first caught Itzler's attention at a 100-mile race. Itzler was running as part of a relay and Goggins was running the full length solo. According to Itzler, Goggins had broken all the small bones in his feet and had kidney failure during the race.
"I'd never seen anything like this in my life, and I was like, 'I gotta meet this guy and see what makes him tick,'" Itzler said Monday on "CBS This Morning."
The entrepreneur called Goggins and went to visit him.
"Five minutes into our conversation, I said, 'You know what? The buckets in my life would be so much better if a little of what he had rubbed off onto me.' And I invited him to come live with my wife and I for a month," Itzler recalled.
He saw determination and grit in Goggins.
"He lost 100 pounds in 60 days to try out, become a Navy SEAL, and became one of the best endurance athletes on the planet," Itzler said.
Goggins admitted he thought Itzler was crazy when he was first approached, but he was convinced that Itzler wanted to push himself more.
But Goggins had one condition before moving in:
"Just do what I say. No matter what," he said.
At the center of Goggins' philosophy is mental toughness.
"The only way you gain mental toughness is to do things you're not happy doing. If you continue doing things that you're satisfied and make you happy, you're not getting stronger. You're staying where you're at," Goggins said. "Either you're getting better or you're getting worse. You're not staying the same."
Itzler chronicles their experience together in his new book, "Living with a SEAL: 31 Days Training with the Toughest Man on the Planet."
"At the time, I was in a routine," Itzler said. "Routines are great, but routines can also be a rut, and I just felt like I wasn't getting better. And I felt like I had to mix up my routine to get better."
Now, what he learned from Goggins still has a lasting impact, he said.
"I found out that we all have a reserve tank, and I found out that I had so much more in my reserve tank," Itzler said.
"Every wall, pretty much a lot of them have doors," Goggins said. "I'm looking for a door to get through it and go to the next limit."
Goggins pointed to his past as the source for his own will power and drive.
"I had a rough childhood coming up, and I just took all that negative energy and made it very positive for myself to drive me. I'm a very driven person. I have passion that almost scares people, just to be successful and make it no matter what," Goggins said.
His drive is evident in his everyday life, as he faces himself in the "accountability mirror" daily and asks, "What have I done today to improve David Goggins?"