The father-of-two was on Wednesday 55km into a 331km or 331,000-step run from the Sydney Opera House to Parliament House in Canberra.

The Outrun 331 campaign started on Tuesday to raise awareness and funds for the Black Dog Institute, which is dedicated to understanding, preventing and treating mental illness.

One in five Australians are affected by mental illness every year, with one of the most common of these being depression

Brendan was diagnosed with depression and anxiety soon after the traumatic birth of his daughter Portia, now 14.

His wife, Suzanne, suffered an abruption, the placenta had separated from her uterus, putting both her life and that of their then unborn daughter in jeopardy.

"My pregnant wife was whisked into the operating theatre as one of the midwives asked, 'Have you both said your final goodbyes?' This question would, in time, trigger a change in our lives forever," Brendan said.

Seven minutes later, his little girl was born tiny and premature, her rough start to life now well behind her as Portia has grown into a lovely teenager.

But Brendan said 30 days after her birth, he was struck down by severe anxiety and clinical depression, the almost-loss of his "two girls" also sparking memories of losing his mum to breast cancer when he was only eight-years-old.

"My system shut down. People were talking to me, but it could have been a different language," he said.

"But there is hope and you can push through. That's what this marathon is all about. It's not a sprint – you have to keep plugging away. It's a journey, and some days you'll walk further than others. You need to take one step at a time, and with others walking alongside you can get through to the other side."

For Brendan, the Black Dog Institute has a special significance. The institute was founded in 2002 – the same year he was diagnosed – and he even owns a black dog, called Boston.

Brendan, who works for the Student Association at the ANU, is doing the run supported by mate and driver Ben Campbell.

Along the way, Brendan is visiting schools and local AFL clubs to tell his story, and to encourage others to share their experience of mental illness. He also plans to collect handprints of people affected by mental illness onto a scroll which will be presented to parliament.

A former player for the Waverley AFL club in Melbourne, Brendan now plays for the ACT in the Masters AFL competition, saying exercise was crucial to his recovery.

"Men in particular don't like to talk about their mental health or things that make them look humble, but it's a conversation we need to have," Brendan said.

"Mental health isn't well understood: you can't just put a cast on it like a broken arm. Resilience plays a huge part in the mind, and when I hit a wall I didn't have that resilience or physical fitness to counter what I was trying to fight.

"It's important to stay connected both physically and mentally. I know that if I didn't take the advice of my psychiatrist in reaching out and reconnecting to people, and in staying physically active, I'd still be stuck where I was."

To support Outrun331, visit http://outrun331.gofundraise.com.au/.