Penrith prop Liam Henry will attempt 10 laps of a 6.5-kilometre route across the Nepean River on October 10.
The challenge, roughly one-and-a-half marathons, will raise money and awareness for Parkinson's disease, which affects his father-in-law Gav.
Gav was diagnosed 13 years ago at 38 and stopped working about six years later as the illness progressed.
"He's an absolute trooper," Henry said.
"But you never hear him complain whatsoever. He doesn't stop, doesn't let it hold him back. He's an absolute legend."
Henry hopes to collect $5000, although improving public understanding of Parkinson's remains his priority.
"I just want people to have an understanding of what Parkinson's is, and how it affects someone," Henry said.
He said Gav experiences dyskinesia, restless legs, freezing when medication wears off, and ongoing back pain.
Henry added that Gav often receives unwanted attention in public because people do not understand his symptoms.
The 25-year-old previously completed a 50-kilometre Blue Mountains hike, which he believes was harder because of its stairs and elevation.
After watching backyard-ultra footage, Henry developed this charity event as a way to honour Gav.
He will rest between laps and has invited others to join him for portions of the run and conversations.
A Penrith victory over Newcastle on Sunday could put the grand final six days before the challenge.
"It wouldn't be the best of prep leading into it," said the Perth-bound forward, who still expects a worthwhile day.