At seven-years-old, he was diagnosed with Ewing Sarcoma, a rare cancer that occurs in the bones or in the soft tissue around bones.

He underwent chemotherapy for more than six months in an attempt to fight it off, and later had surgery to have the cancer removed, but nothing worked, and doctors were forced to amputate his right leg below the knee.

"At the time I thought I dealt with it OK, but I actually felt different to everyone else," he said.

"I felt physically below everyone else and that I was going to be susceptible to a lot of bullying, so I went on the counter-attack and was a bully myself."

For years, Mr Twomey struggled with unresolved grief over the loss. He says he never talked about it or learnt to be OK with it, and felt that he was being held back from completely enjoying his life.

It wasn't until 2016 that the now 23-year-old received a grant from the START Foundation for a new prosthetic leg, which he says dramatically changed his trajectory.

"When I applied, I applied telling them that I'd go to the Paralympics if I got the leg, which was a bit of an overreach but I thought it would look good on the application," he said.

"When I got the leg, one of the [START Foundation] board members told me 'well, you've got the leg and the Paralympics are in a couple of years so get training'.

"I decided that, these people believed in me enough to give me a leg, I might as well actually go for it."

In the years since, Mr Twomey – now a children's swim instructor living in Bondi – has been working up to the 2024 Paralympics in Paris, and The Sun-Herald City2Surf, presented by Westpac, is on his training agenda.

Not only is the event a great physical challenge, he says, but an opportunity to raise money and awareness for those affected by brain cancer, which is one of the most common cancers affecting children.

"I had cancer when I was a kid and it's scary to see how what happens to a child can also affect the people around them," he said.

"I think it's important that we raise, if not funds, awareness [about brain cancer] and keep people talking about it."

The money he raises from the event will be donated to the Cure Brain Cancer Foundation, but Mr Twomey stresses that he thinks of it as no noble feat – he's just someone who's trying to "do his best and do some stuff for others".

"Being physically active helps me mentally, it helps me integrate with the community, it helps me enjoy life – I get a whole new lease on life every day when I do it," he said.

"I know I've had a bit of hardship and some struggles and this and that, but ... anyone can do that, anyone can decide that they want to sign up for the City2Surf and run for brain cancer to create a bit of money and awareness [for their chosen charity]. I'm not Superman."

The City2Surf will be held on Sunday, August 12. Register online.