Don't be questioning the lengths that Kurt Fearnley and British star David Weir will go to win Sunday's Paralympic marathon in London.

Fearnley, who is chasing an unprecedented third title in the T54 wheelchair race in central London, has crawled the Kokoda Track.

The intensely-driven Weir has been untouchable on the track at his home Paralympics and is hungry for a fourth individual gold medal of the Games in front of his adoring fans.

The great rivals rarely clash in the gruelling event but when they do, it tends to be captivating viewing.

"We get along well off the track but on the track it is a bit fierce," Fearnley said.

"I remember one race Dave and I spent racing each other and we both threw up during that marathon and threw elbows and clashed a bit.

"But as soon as you come off, you tap gloves, you take the helmet off and go back to reality."

Fearnley says he doesn't mind the steamy conditions forecast for Sunday with the race starting just before midday.

The 31-year-old hopes his rivals wilt as the heat bounces off the bitumen with the thermometer expected to hit 27 degrees.

"The marathon is a bit of a test in your pain threshold and if that tips someone over, I will be quite happy," he said.

"It is all about managing whatever things are thrown at you over 26 miles.

"The road surface, taking a corner a certain way and avoiding the barriers and eventually trying to be as fresh as possible for the last 200 (metres)."

The flat course does not suit Fearnley with a bunch sprint expected.

However the Australian has an ability to lift for the marathon and Weir admitted his programme was catching up with him.

"I'm tired," he said after his 800m win.

"I probably seem like I'm not enjoying it but I am tired now. I need to recover."

Fearnley's racing has been up and down, describing his performances as the "good, bad and ugly."

He won silver in the 5000m, finished seventh in the 1500m and was bundled out in the 800m heats.

The Australian was rich in his praise for six-time London marathon winner Weir but found it more difficult to express what a win on Sunday would mean for him.

"I have the opportunity to be undefeated for a decade in Paralympics and at world championships," he said.

"Even if I am unsuccessful it is going to be a positive experience ... it is hard to describe the potential of how amazing Sunday can be."