"I said let me think about the challenge I want to have," he recalled. When he saw the New York marathon on television last November, "I said, hmm, that looks like a great challenge. Something very impossible to do, which is something I want to attempt to do."
The New York Road Runners helped Mr. Walcott enter the marathon, according to his communications director, Natalie Ravitz. Most people who are not elite athletes enter via lottery. "They were very gracious," she said.
Mr. Walcott started training in earnest nine months ago.
"I was never really much of a runner," he acknowledged. He said he would typically use the treadmill at his local Y.M.C.A. in Queens about three times a week for half an hour, "and that was walking and running."
Mr. Walcott was a deputy mayor before he took over as schools chancellor in April. He said he found time to train in his busy schedule by waking up earlier and running the longer distances on weekends. His longest run was 22 miles, but he says he did it too late in the day (after a few events as chancellor) and had a tough finish. He ran the Staten Island half-marathon in October in 1:51:53, finishing 10th in his age group.
"Each run has been a lesson learned," he says. "I've now learned the beauty of the gel packs and hydrating."
Mr. Walcott said city students have been his biggest supporters. He often mentions the marathon when visiting schools. He ran with the girls from Port Richmond High School. He also talked with schoolchildren at the annual Run With Champions event in Central Park this week.
What do the students tell him? He said their main advice has been: "You can do it. They've been plain and clear: we know you can do it."
The chancellor said he did not have any goal for Sunday's race beyond finishing. He stopped running more than two weeks ago to give body time to rest and relax. His weekend plans included grocery shopping and hitting the dry cleaner.
"If I do anything, it will be just little quick burst of runs along the block and in the neighborhood, but nothing long at all," he said on Friday.
As schools chancellor, he said he hoped his race would send a message to the city's one million students.
"Exercise is important; health is important," he said. "Healthy eating is important. And setting goals are important as well. ... Even if you don't make your goals, it's important to set goals."
Just don't ask him about his next big birthday plan. He said he would not think about that one for a while after the marathon.