GEOJE, South Korea — SIM JAE-DUK, a South Korean shipyard worker, checked into his motel in Front Royal, Va., on a May evening in 2006, sleepy after his first-ever trip to the United States — on a budget flight that first stopped in Tokyo and San Francisco. Two days later, he set off on the Massanutten Mountain Trails 100-Mile Run and won it, setting a record of 17 hours, 40 minutes, 45 seconds, which has yet to be broken.

The next day, he caught a flight home.

“I don’t like missing work because of my running,” said Mr. Sim, 44, who works at Daewoo Shipbuilding and Marine Engineering on this island off the southern port of Busan.

Massanutten race officials and runners recall him as a “total unknown” who spoke no English except “Water, water!” and “Thank you!” But Mr. Sim has become something of a legend among South Korea’s amateur marathoners, whose population has exploded in the past decade. He has been nicknamed the Korean Forrest Gump, after the movie character who runs across America.

Mr. Sim got his start running after six years of nine-hour workdays inside the ships, breathing chemicals and dust through a face mask. His respiratory system was so weak that in 1993, doctors recommended surgery to help him breathe. “Because of breathing difficulties, I always kept my mouth open, looking like an idiot,” he said. He also lost most of his sense of smell. (On race days, he asks fellow runners to smell his lunch box to check if any food has gone bad.)