02/11/2015 2:23 PM AEDT | Updated 15/07/2016 12:50 PM AEST

Indigenous Man John Leha's 'Amazing' New York Marathon

Supplied/Indigenous Marathon Project

John Leha once weighed in at almost 170 kilograms.

Six months later, and 50kgs lighter, the young Australian of mixed Indigenous and Tongan heritage just finished the New York Marathon - one of the world's toughest road races.

"I'm on top of the world," Leha told The Huffington Post Australia after competing as part of the Indigenous Marathon Project, a program headed up by Aussie marathon legend Robert De Castella.

"I'm so full of emotion and proud of myself and my squad and the Indigenous Marathon Project for getting me to this point," Leha told HuffPost Australia from New York, where he's recovering after the 42-kilometre slog.

"It's a special thing to do a marathon in your life and I'm so lucky to have been able to push through the struggle to be able to do that.

"After the 30-kilometre mark I really felt the pain in my muscles, the aches in my joints, and it was hot here, very different to what we've been training in."

For Leha, the six-month training program, which included a 30km trial run in Alice Springs, has been a journey of healing.

The 30-year-old Sydneysider has experienced depression and binge eating, dating back to when his 24-year-old brother Tohi died from rare form of cancer.

He said pounding the pavement and fixing his diet has helped turn his life around.

"It has shown me that anything is possible. Even despite all the things that went wrong in my own life, I'm strong enough to do anything I set my mind to," he said.

"It's the biggest and best thing I've ever achieved in my life."

The Indigenous Marathon Project, founded in 2009, uses running to "create and inspire" Indigenous leaders.

Counting Hugh Jackman among its supporters, the program also promotes healthy living in Australian Indigenous communities, and works to cut down lifestyle diseases.

Fifty-three Indigenous Australians have so far graduated the program, which also includes studying for fitness qualifications, and a number of fun runs.

Leha said, for him, the massive effort was a personal message to his family and his communities about what's possible with good diet and exercise.

"Indigenous people struggle with lifestyle diseases like heart attacks and diabetes, and in the Tongan community rugby league and union are the predominant sports," he said.

"You grow up thinking things like 'you're an islander so you're supposed to be big', or 'you're an Aboriginal you're supposed to do this', I'm so happy I can break that down."

Leha's words of wisdom: "A simple thing like running can change a person's life.

"I haven't had an opportunity to give them a call yet, but I know my parents are extremely proud of me," he said.

"At my weight, over 120kg, to be able to go out there and run a marathon, I can't even begin to understand the thoughts that are going through their heads."

De Castella, who has been with the program since it began, told The Huffington Post Australia was incredible to see Leha "tough it out" on Monday.

"He settled into a pace he believed and knew he could keep going," said the former marathon champ, who won two Commonwealth gold medals for the event, and dominated for most of the 1980s.

"How far he's come in his self confidence is amazing, he realises through his story he has capacity to help so many people."

The man known as 'Deeks' said all 10 of his runners made it across New York's finish line in hotter and more humid conditions that the team trained for.

"They all dug deep and finished and did really really well," he said. "There's a lot of emotions, they've worked so hard over the last six months to prepare for this."

Back in his room, Leha was eager to get stuck back into training, after a bit of rest.

"I am lying down in my bed as we talk, it's hard to move," he said.