When Thad Ghim lines up in Hopkinton, Mass., for the start of the 116th Boston Marathon, the retired Emory medical school professor and pediatric oncologist won’t hold much hope for placing high.

For one thing, Ghim doesn’t train heavily for a marathoner, typically running no more than 30 miles a week. Moreover, Ghim, 73, is on the high end of his 70-74 age group.

The 70-year-olds, Ghim said, are “spring chickens coming up.”

Ghim, an Atlanta resident, can find satisfaction simply in qualifying for Monday’s Boston Marathon, arguably the most prestigious marathon in the world. Only 11 percent of U.S. marathon finishers qualified for this year’s race, according to Running USA, a running industry trade association.

The oldest of 355 Georgians registered for the race, Ghim also falls into a presumably one-person subset – three-time Boston entrants who got into running at the age of 47 after a near-death experience, who directed the pediatric oncology and hematology departments of two major Korean hospitals and who have offered guidance and inspiration to a younger generation of runners.

“He is such an amazing example,” said Sunjune Lee, a 52-year-old Johns Creek resident who will run her first Boston Marathon Monday after taking up running about four years ago with Ghim’s help.

Ghim had no such aspirations one stormy Sunday morning in 1987, when he took his only son Michael to Stone Mountain for their weekly hike. Thunder and lightning chased them down the mountain, and by the time they reached the bottom, Ghim could hardly breathe. He remembered how the skies appeared yellow to him. He lay down on a bench and, until he recovered, thought he was dying.

“From then on, I was thinking, I’m not healthy,” said Ghim in an interview last week.

Ghim, who came to the U.S. in 1966 from Korea for a residency at Harvard Medical School and then moved to Atlanta in 1978 to teach and practice medicine at Emory, didn’t have much time for exercise. But he dusted off a pair of sneakers from the basement and began to run in his northeast Atlanta neighborhood, at first less than half a mile.

After a few years of jogging, his wife Heather suggested he enter a local 5K. He placed third in his age group. Over time, he moved to longer distances, eventually becoming a frequent age-group winner in half-marathons. He said he became motivated to try the 26.2-mile distance after hearing that Home Depot co-founder and Falcons owner Arthur Blank, a few years younger than Ghim, ran marathons.

He said he ran his first at age 53 and has run 29 total. An Atlanta Track club member, Ghim estimates he has run more than 50 half-marathons with a personal best in the 90-minute range. His fastest marathon was about 3 hours, 50 minutes. His son Michael, 37, is proud of his father’s accomplishments, though he acknowledges the strangeness of thinking of his dad as a jock.

“I laugh with other people about it, how he’s more athletic than I am at this point,” said Michael, an emergency-room doctor in Elon, N.C.

Ghim made time to train and compete even during the 11 years he lived in Korea overseeing pediatric hematology and oncology departments at the Asan Medical Center and then the National Cancer Center of Korea. He imported the American fund-raising practice of sponsoring marathoners to raise thousands of dollars for children’s cancer charities in Korea.

Monday’s race won’t match his most cherished running memory – returning to the hospital from his leg of the Olympic torch relay in 1996, and conducting his own relay among young cancer patients, whose smiles and delight Ghim can still picture.

But it will be a personal relay of its own. After returning from Korea in 2008, Ghim was invited to join an informal running club for beginners. At the group’s Saturday morning runs at George Pierce Park in Suwanee, Ghim has shared his knowledge, good humor and example.

Ghim’s qualification for the 2010 Boston Marathon inspired the group , mostly running newcomers in their 40’s and 50’s, to dream it for themselves. Monday, the field will include six club members including Ghim and Lee, who four years ago couldn’t even run three miles. Lee calls Ghim an inspiration and a hero.

Speaking last week, Ghim said this will probably be his final marathon before re-considering.

“I don’t know,” he said. “We’ll see what happens in Boston.”

Limitations do not become him.