(Note: If you want many more lessons from Navy SEALs, my colleague Bill Murphy, Jr. has written the ultimate guide.)

Fact: 99 percent of people who start marathons finish them.

What are we to make of this incredibly high success rate? As anyone who has ever run a marathon can tell you, the correct takeaway isn't that running 26 miles is a breeze. Everyone hits a wall of pain at some point, yet almost everyone keeps going.

Instead, the real lesson, according to entrepreneur and endurance athlete Jesse Itzler, "is that we have so much more in our reserve tank than we think we do."

This is a truth Itzler learned in an unusual way -- by inviting a Navy SEAL he met while running a 100-mile race (Itzler was doing it in relay, the SEAL was doing it alone) to come and stay with his family for a month and teach them the secrets of incredible mental toughness. He shared one of his biggest lessons from the experience recently in a short video for Big Think.  

The 40 percent rule

It's all well and good to tell people they are capable of accomplishing way more than they believe they are, but when the going gets tough, vague reassurances probably aren't going to count for much. That's why Itzler's unusual houseguest offered something more specific than empty encouragement. He taught Itzler the Navy SEALs' "40 percent rule." (Spoiler alert: He also made him do a lot of pull-ups to illustrate it.)

"He would say that when your mind is telling you you're done, you're really only 40 percent done," Itzler explains. "And he had a motto: If it doesn't suck, we don't do it. And that was his way of every day forcing us to get uncomfortable to figure out what our baseline was and what our comfort level was and just turning it upside down."

It turns out that the 40 percent rule is why so many people are able to finish marathons. When you hit that wall, you're really only 40 percent through your stores of energy and determination. When your body complains, your will still has a lot to give. And as the success rate of runners makes clear, that's true of just about everyone.

That's handy to know if you suddenly feel like your legs are going to fall off at mile 18 of a marathon, but it's a truth that can have a huge impact even if long-distance running really isn't your thing. Whenever life puts a challenge in your path and you feel like you're on the edge of giving up, you can lean on the SEALs' 40 percent rule to remind you that your apparent limits really aren't.

"We all have that will. It's just a matter of how we apply it not just to the once-a-year marathon, but to a variety of things in our daily lives," concludes Itzler.