Originally planned for 2020, the trip was delayed by the COVID-19 pandemic.

But this weekend the Canberra federal MP will finally take on the gruelling 90-kilometre course from Pietermaritzburg to Durban to raise money for the Indigenous Marathon Foundation.

Comrades is the world's oldest and largest ultramarathon, founded by a World War I veteran in 1921 to commemorate South African soldiers killed in the war. It's also known as the Ultimate Human Race and has a 12-hour time limit.

"It's got that sort of harshness about it that you might expect from a race designed by a group of military veterans but it's also got a whole lot of wonderful, inclusive values - strength, perseverance, and ubuntu, which sort of translates as spirit," Dr Leigh said.

"All of that seems very fitting for an Indigenous Marathon Foundation linkage."

The Indigenous Marathon Foundation was established by former world marathon champion Robert de Castella to create opportunities for First Nations people through running and physical activity.

Dr Leigh said de Castella was a childhood hero, who has been giving back to the sporting and Australian community since his retirement.

"In the year in which there's a lot of conversations around reconciliation in the context of the Voice referendum, Rob has been a really important leader," he said.

"We hosted the [Indigenous Marathon Foundation squad] in Parliament here recently and to hear those young men and women tell their stories is remarkable, tales about working through mental health challenges, violence in the communities, challenges with housing overcrowding.

"Many of the things that we know to be true in Indigenous communities, but to hear how young Indigenous leaders are overcoming that and becoming an inspiration to others is fabulous."

Dr Leigh has been supporting the charity since 2015, running all World Marathon Majors including in New York, Berlin, Tokyo and London to raise money for the foundation.

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His marathon journey has not only allowed him to follow the footsteps of running legends but also his father and grandfather.

"It's in the genes and I feel lucky to be able to have a sport that I enjoy and I can do reasonably well," he said.

Having now run more than 25 marathons, Dr Leigh no longer harbours fears about whether or not he will reach the finish line.

"My first marathon was the Canberra Marathon and I remember almost crying at about the 40-kilometre mark when I suddenly realised that I didn't have to fear finishing the race, that I actually was going to finish the race," he said.

Now, he says, the question is "much more about pace".

"Any new race has both the excitement of being able to finish well, but also the trepidation of not knowing the course and not knowing how you need to go and pacing yourself. But that's the joy of a new race."

Dr Leigh will be running the Comrades ultramarathon on June 11 and has so far raised nearly $10,000 for the Indigenous Marathon Foundation.

He will be covering his own travel expenses.

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