Think for a moment about the highest mountain you ever climbed, the deepest valley you ever crossed, or the longest river you ever had to swim. Perhaps it was healing a relationship with an estranged loved one, closing a big deal at work, overcoming an addiction, surviving an illness, succeeding against the odds or making what seemed like an impossible dream come true. I imagine that there were moments in the process when you felt weak, became convinced you could go no further, and wanted to turn back and give up.

A few years ago I decided to walk in an 18-mile marathon. At about the 10-milke mark, I hit the proverbial wall. Every muscle in my body ached, and I hurt in places that doctor