About 99 percent of the people who start marathons in the United States finish them. That’s an astoundingly high number considering the pain and turmoil that every marathon runner faces. What each runner has in common, says author Jesse Itzler, is that they hit a wall where their mental resources are exhausted. At this point, sheer physical will maintains their strength — and this is the will that everyone has, but we seldom know how to tap into it.

Itzler’s way to break through his own mental barriers was to invite a Navy SEAL to live with him and his family for a month. First item on the agenda? Doing over 100 pull-ups. The lesson wasn’t about physical fitness, but about mental fitness and how we each have an unused reservoir of strength and determination inside of us.

Itzler is the author of Living with a SEAL: 31 Days Training with the Toughest Man on the Planet and/or his site.