A team of six are preparing to visit every one of the 270 London Underground stations in a day.
The challenge will start at 5.02am from Hounslow West tube station on April 15 before a race against time to see if they can visit every station on the 11 tube lines before the last train at 11.30pm.
Simon Cox is leading the challenge, which he has already attempted twice but not yet completed.
He said: “I did it in about 1995 but didn’t get round all of them.
“I got to 256 out of 270, and the last time I did it I only made 120, so it’s bugging me now.
“This time I have got a little bit of help.”
Simon will be joined by Ian Byers, David Antino and Phil Goulding.
And they will also have two members of Team GB helping them in the shape of Kelly Edwards, a judo star who competed at the London Olympics in 2012 and won a silver medal in the 2014 Commonwealth Games in Glasgow, and Tom Bosworth, Britain’s number one for race walking.
Simon said: “The six of us all have different backgrounds but I think we will get each other through it.”
The team has a route planned out, but they may have to think on their feet if problems such as signal failures, breakdowns or leaves on the line arise and they have to find an alternative way round the 250 miles of track.
I did it in about 1995 but didn’t get round all of them, so it’s bugging me now.Simon Cox
Simon said: “It only takes one breakdown and the whole thing is gone.
“I have got good knowledge of the network, but the reality is are we fit enough to take it on?
“I think the only thing you can do is make sure your brain is awake because anything can change.”
The team will be eating plenty of carbohydrates the day before and will be carrying food and energy drinks to keep them going during the day.
All donations for the challenge will go to Niamh’s Next Step, the Wellingborough-based charity set up in memory of five-year-old Niamh Curry who lost her fight against neuroblastoma in May 2012.
Her parents Sam and Chris Curry launched the charity to raise awareness and fund research into the rare childhood cancer.
Simon, who visited the four national stadiums of England, Wales, Scotland and Northern Ireland last year for the charity with Chris Curry, said: “I have got to know Sam and Chris over the last few years.
“I admire what they have done because I know they are still going through grief and it’s a tough thing for them to go through, but they don’t show it.
“They are doing something positive when it is probably the worst thing you could ever face in life.”
More than £460 has already been raised of the £1,000 fundraising target set by the team.
But there is still time to sponsor them or sponsor your favourite station for £5.
For more information about the charity go to www.niamhsnextstep.com.