"The last 4.2 miles it was complete emotional, mental and physical warfare," Herndon told WCBS 880's Marla Diamond. Photo credit Brian Fluharty-USA TODAY Sports
After his legs gave out, Herndon would not give up. He kept repeating the names of three of his friends killed when a 400-pound IED exploded under the convoy he was leading in Afghanistan in 2010.
"Nothing compares to the pain that they went through and what their families went through," he said.
Marine Mark Juarez and British war journalist Rupert Hamer were killed instantly. Marine Matthew Ballard suffered injuries so painful, he took his own life.
"They put their trust in me and I failed them," he said. "I write their names on my bibs and my wife writes their names on the backside of my hands."