The former Rhinos captain will run 101 miles, from Leicester - where he now works as an assistant-coach for the Tigers rugby union club - to Leeds in just 24 hours, beginning on Monday morning.
The ‘extra mile challenge’ comes a year after Sinfield raised more than £2.5m for MND research by running seven marathons in seven days.
Including sponsorship, Sinfield has already been pledged around £50,000 more than his initial £100,000 target, which will be split between the MND Association and Leeds hospitals’ charity appeal for the Rob Burrow MND Care Centre in the city.
The 41-year-old led Leeds to seven Grand Final wins in an 18-year playing career and is the third-highest points scorer in rugby league history, but admitted he is “not quite sure" if he is up to his latest feat of endurance.
The run will be split into seven-mile sections, with an hour’s break between each leg.
His longest run so far was a 12-hour training session last week, which covered 52 miles.
Burrow’s plight has captured public attention nationwide since he was diagnosed with MND, which is a terminal illness with no known cure, in December, 2019.
Sinfield revealed his former teammate “just burst out laughing when I told him, which was the response I expected”.
He said: “We are running on behalf of Rob, but for two great charities Rob is so passionate about.
“That is really important to all of us; he is certainly the big motivator in this, the inspiration.
“We want to try and do our best for him and all the others suffering with MND.”
Sinfield had never run more than the marathon distance of 26.2 miles before beginning training, but initial plans for a cycling challenge or long walk were rejected as “not hard enough”.
He said: “It was my wife, Jayne, who came up with the idea of how far is it from Leicester to Leeds, why don’t you run from rugby club to rugby club?”
“I do feel like we are winging it a little bit.
“When I started to research a 100-miler, a lot of the research is you need six to nine months’ preparation to step up from the marathon distance.
“I had never run further than a marathon before, so we gave ourselves five-six weeks.
“I do understand a big part of it is mental; there’s certainly some physical parts of it that are going to hurt and be horrible, but mentally it is going to be tortuous at times and emotionally it is going to be tough.”
Two friends, David Spencer and Chris Stephenson, have volunteered to run the full distance with Sinfield, while former teammates, potentially including Jamie Jones-Buchanan, Jamie Peacock, Danny McGuire and Gareth Ellis, could join him for sections of the course.
“We want it to be tough and challenging, people don’t support it if it isn’t,” Sinfield added.
“I am not quite sure I will be able to do this, but I will do everything I can to get through.”
Donations can be made via https://donate.giveasyoulive.com/fundraising/kevin-sinfields-the-extra-mile-challenge.
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