There was an odd sound as Kit Crosby watched her father's plane disappear below the line of trees. It was a sight she and her mother had become used to; after semi-retiring from a career in the financial industry, J. Player Crosby volunteered his amateur flying skills around the world, doing everything from ferrying Native American athletes to ultra marathons in the west to tracking elephants in Africa.

Itsing Batsholelwang, left, and his twin brother Tirafalo warm up in Manhattan before a training run for Sunday's New York City Marathon. Philip Montgomery for The Wall Street Journal

After every trip he would land softly in the field behind their Massachusetts home, mischievously buzzing the cows that grazed nearby. This time was no different, her mother assured her.

Then they saw the smoke rising from above the trees. The two women hurried into their car and drove down to the field just in time to watch the plane explode.

Mr. Crosby had suffered a massive heart attack during the landing, just a month before he and his wife had planned to move to Botswana full-time to help improve conditions in the country. Within a week, friends had donated $50,000 to their cause without being asked.

"People said, you've got to continue his dream," said his son, Jay Crosby, a 37-year-old middle school history teacher and sports coach at Riverdale Country School in the Bronx. "And so Somarela was born."

On Nov. 7, Mr. Crosby will lead a group of 17 runners at the ING New York City Marathon in raising money for the Somarela Fund, which supports children in Botswana who have been sexually abused, suffer from drug or alcohol addiction, are orphaned or homeless. Their main partner in Africa, the Bana Ba Letsatsi (BBL) center, currently works with more than 230 such children in Maun.

When Mr. Crosby learned that two of BBL's students, identical 20-year-old orphans named Itsing and Tirafalo Batsholelwang, were accomplished runners he invited them to join the team at the marathon this year, along with one of the center's instructors, Paul Moleseng.

Just five years ago, raising money through the New York City marathon would not have been possible. The race inaugurated its charity field in 2006, when runners raised about $11 million. This year, New York Road Runners is hoping to raise $26.2 million from 7,400 charity runners across more than 100 organizations.

Somarela participated for the first time last year, fielding 20 runners who raised $40,000 through pledges from friends, family and associates. Mr. Crosby is hoping to surpass that number this year to support the construction and operation of an orphanage for the BBL children without homes or who live in danger. Several young female children have been raped; others have become prostitutes or exposed to violent relatives, including witnessing murders.

"Unfortunately it' s a common occurrence," Mr. Crosby said.

The twins would have benefited from such a center when they were young. Instead they will run to support the next generation of orphans in their country.

They arrived in New York on Saturday and have spent the week doing training runs and sightseeing, marveling at the spread of autumnal colors along the Palisades, the steel of midtown skyscrapers, the crush of people in Times Square. It is a stark contrast to the low-slung Maun landscape.

"I always liken Maun to kind of a tombstone in that it's a very dusty middle of nowhere kind of place," said Mr. Crosby.

"Donkeys everywhere," Mr. Moleseng said. "More donkeys than people."

Also feral dogs, Mr. Crosby added, and the occasional wildlife that wanders down from the delta; he has spotted hippos waddling through town along with zebras and giraffes.

In Mr. Crosby's Brooklyn home, Mr. Moleseng and the twins have eaten their first bagel, watched their first morning talk show (Kathie Lee and Hoda) and adjusted to the different running conditions as they prepare for the marathon.

Last Sunday, the twins ran the last 10 miles of the course, finishing in just over two hours, well ahead of Mr. Crosby and Mr. Moleseng. Afterward they moved into the sun, shivering from the unfamiliar cold, despite very little sweat slicking their bodies.

The course's hills had caused little problem, even though Maun is notoriously flat. Trickier was the solid pavement that made the boys' bodies shudder with every step; in Africa, they frequently run on sand.

But other challenges—like frequent interruptions at traffic lights or worries about getting lost as they dashed through the streets—will not be a problem on marathon Sunday, Mr. Crosby and Mr. Moleseng assured them.

Other differences were a delight: Mr. Moleseng liked the blanket of reddish leaves lining the pathway into Central Park, cushioning their steps. Running beside other joggers, cyclists, roller-bladers was also a welcome change.

"It's very nice when you're running and you see so many other people doing what you're doing," Tirafalo said, with Mr. Moleseng translating. "Normally in Maun when you're running you're the only one and people are like, what's wrong with him? But here everyone else is doing it and you feel normal. It encourages to see you're not the only person who loves it."

They were stunned when strangers cheered them on during their training run, powering them through any sluggish moments. "That gives you courage," Mr. Moleseng said.

The twins have needed a lot of that over the years. They were orphaned when their mother died shortly after their birth. After moving in with an aunt, she soon died, too. Finally their father, who worked as a safari guide and was gone for long stretches at a time, took them to stay with him in a one-room house in Maun. Several outbuildings on the property were made of mud and washed away during rains.

There was no electricity, plumbing, or privacy. Meals, frequently just a heap of cornmeal, were cooked in a firepit outside. When their father left to travel into the bush, sometimes they would not eat for days.

"It was very difficult when dad was not around," Itsing said. "Very difficult."

In 2004, their father was unexpectedly sent home. He, too, had fallen sick. A year later he died from AIDS.

He was not alone: Botswana has one of the highest HIV/AIDS rates in the world.

Mr. Moleseng said he has two aunts and a cousin who died from HIV and AIDS. "A lot of close relatives," he says. "Lots of friends."

The twins still remember their father. He wasn't very strict, they said. But "if you're lazy, then he didn't like that," Itsing remembered, sitting in the faculty lounge at Riverdale Country School and speaking quietly. "You had to work. If you want to get something out of life you've got to work. That's what he told us."

Through BBL's support, the twins are studying plumbing at a technical college where they hope to graduate next year. Meanwhile, they continue to run.

Already they have competed internationally in the 200-meter and 400-meter distances—but their aspirations are greater.

"We want to see ourselves representing Botswana in the Olympics," Tirafalo said.

They have just one immediate goal, though, Mr. Moleseng told an assembly of Riverdale Country School students this week— including dozens who have traveled to Botswana over the summers to help build playgrounds, bathrooms and gardens.

"We're going to beat all the Kenyan guys," he said with a big grin.

Write to Sophia Hollander at