This time last year, Leigh Chivers was married with two young sons.

Today he's a single dad of one.

In March last year, Mr Chivers' wife Sara got the devastating news that her brain cancer had returned after eight years in remission.

Six months later, the devastation turned to disbelief as the couple's 18-month-old son Alfie was also diagnosed with brain cancer.

It was a completely different type, but it too was malignant, aggressive and terminal.

Both Sara and Alfie underwent surgery. But just before Christmas Sara went into palliative care, and in January she lost her battle. Five months later, baby Alfie died.

Leigh and Sara Chivers with their sons Hugh and Alfie. ( Supplied: Sara Chivers )

Few people could imagine the extraordinary tragedy and grief Mr Chivers endured.

"I just hit a wall where I couldn't see a future any more," he told 7.30.

"I couldn't look beyond a week. I think it was just too scary. I just couldn't."

He said the last two years had given him a sharp focus on what was important.

"It makes you realise how precious the time you have with your family is, how precious the time with your children is, and how all our time is finite," he said.

"Really all we've got is the moments that we have with each other.

"I think it's just been about survival. If I just focus on how hard it's been or how tragic it is all the time, I don't think I'd get out of bed to be honest."

'Sara has orchestrated the whole thing in a way'

Leigh Chivers says Sara encouraged him to get on with life. ( ABC News: Lauren Day )

Mr Chivers did continue to get out of bed, largely for the sake of his four-year-old son Hugh.

He not only picked himself up, he threw himself head-first into fulfilling a promise he'd made to Sara before she died.

"She encouraged me to get on with life and do the things that I enjoy," he told 7.30.

"And one of those things was, she said she wanted me to do the Hawaiian Ironman, you know, for myself but also for her.

"That was something we never got to do and she knows that it was a dream of mine to do, so it gave me that extra motivation to do it, finally."

Ironman triathlons consist of a 3.8-kilometre swim, a 180km bike ride and a full 42km marathon.

Even by those standards, the event in Kona is notoriously gruelling, and tough to qualify for.

Leigh Chivers watches as Hugh jumps in. ( ABC News: Joshua McDonald )

The last chance Mr Chivers had was the Cairns triathlon, a week after Alfie passed away.

He failed to finish. But when the Hawaii Ironman organisers heard his story, they offered him one of two honorary places.

"I was pretty shocked and I was blown away as well," he recalled.

"It just all came back to me how much Sara orchestrated this whole thing in a way, or she's gifted it to me somehow."

Sara's mother Helen Clark said her daughter would have been thrilled for Mr Chivers.

"It was her dream for him, so I know she'll be there with him and so will Alfie," she said.

"They'll be there all the way with him. So yes, we're just so proud that this wish has come true and this dream for Leigh as well."

'At the finish line of two impossible years'

Sorry, this video has expired 7.30's story about Sara Chivers and her family from November 2017.

While more people than ever are surviving other forms of cancer, brain cancer survival rates have barely improved over the past 30 years.

Ms Clark said her daughter used her final months to draw public attention to the issue.

"Sara was a selfless person. It was always about everyone else," she said.

"Especially once Alfie got diagnosed, she just said we have to do something about this, and she has done and it will continue."

Now Mr Chivers is taking up that fight.

Leigh Chivers is in training for the Hawaiian ironman, something Sara dreamed he would do. ( ABC News: Lauren Day )

"Anything we can do to raise awareness is worth doing, and it's worth doing because that's exactly what Sara did," he said.

"She was so heartbroken and angry about what happened to Alfie and herself that she just wanted to use her story to try and further the cause and try and improve the future for children like Alfie and for people like Sara."

He has had to cram about 12 months of Ironman preparation into eight weeks, and fit it around working and looking after Hugh.

When he lines up in Hawaii on October 13, he won't focusing on big results or a record time — just finishing the race, and what his wife started.

"I can't wait to see [Hugh's] smile and I can't wait to give him a high five and a hug and just get to the finish and just enjoy it with Hugh [and] with my whole family," he said.

"We'll all feel like we're at the finish line of two impossible years."

Watch Lauren Day's story about the Chivers family tonight on 7.30.

Donations to brain cancer research can be made at the RMH Neuroscience Foundation and Robert Connor Dawes Foundation.