Actor Edward Norton already wanted to be in better shape for his 40th birthday than he was on his 30th when the idea hit — why not join members of an African tribe famous for its runners and run the New York marathon?

“Incredible Hulk” star Edward Norton, second from left, has trained for a marathon with Maasai tribesmen in Kenya

Before he knew it, the star of “The Incredible Hulk” and “Fight Club” had signed on to the effort, despite never having run a marathon before.

“The idea picked up traction pretty quickly,” said Norton, who turned 39 in August. “Then, I was like, ‘Wait a minute. What have I just done?’”

The run is a benefit for the Maasai Wilderness Conservation Trust, an organization working to promote sustainable development while protecting the east African grasslands that the Maasai tribe calls home. Both the traditional tribe’s way of life and its environment have been threatened by rapid industrial development.

Norton discovered, and became a supporter of, the trust about 10 years ago during his first visit to Kenya, where he met founder Luca Belpietro.

“I was impressed at the way he was really genuinely working with the community to create value out of being good stewards of the land,” Norton said.

Belpietro will be one of nearly 30 runners joining Norton on the Nov. 1 marathon. Also running will be magician David Blaine and three Maasai tribesmen.

The Maasai are at the heart of Kenya’s long tradition of distance running. Norton said one of the runners, safari guide Parashi Ntanin, is so fast he could probably leave the rest of the group behind and run the race in less than 2½ hours.

Norton said he’s run as much as 15 miles twice during his training for the 26.2-mile marathon and that despite some leg and knee pains, he’s confident he’ll finish the endurance race.

“I’m really enjoying it,” he said after a workout Wednesday in New York. “I’m enjoying having a goal.”

A competitive rower at Yale, Norton said he’s experienced the so-called “runner’s high” during his training, which fans can follow on his Twitter account, @EdwardNorton.

“Running in the park, I got a taste of what people say about the New York marathon. There were people of every shape and size out there,” he said. “I really got a lift out of it.”

Norton’s latest film, the independently produced “Leaves of Grass,” is set to premiere this week at the Toronto International Film Festival and his company’s “By The People: The Election Of Barack Obama,” a documentary about President Obama’s historic campaign for the White House, debuts on HBO on November 3.