He's hoping to break the world record to raise money for youth mental health initiatives, an issue he holds close to his heart.

With the current record standing at 30 hours, Johnston is aiming to surf for a whopping 40 hours and catch 500 waves by the time he paddles into shore on Friday evening.

The 40-year-old former pro surfer and long-distance runner faces possible blindness, infected ears, dehydration, sleep deprivation, hypothermia and drowning - but says he is prepared for the gruelling challenge.

"I thought I could just do it. I can run for forty hours," Johnston said.

"But, this way, I can surf with people, bring in the community and make a difference for the future."

A decade ago, Johnston's father took his own life, fuelling his effort to shine a light on youth mental health and suicide prevention.

He hopes his world-record effort can raise $250,000 for the Chumpy Pullin Foundation and other initiatives to help young people with their mental fitness. Nearly $200,000 had been donated by Thursday afternoon.

The father of two says he has faced many of his own battles and wants to inspire the kids he now coaches in surfing and provide them with practical tools and support to make it through tough times.

"I'm not nice to myself at times. I put a lot of pressure on myself to be a great husband, father and friend," he said.

"I tell myself, 'You're hopeless.' I'm pretty mean.

"I push myself to the limits with my adventures to prove to myself I'm worthy and can get through hard times. That's when my lessons are learnt."

Australian Associated Press