For most of us the thought of running 100 miles is a challenge in itself.
But for one Co Tyrone man 100 miles simply isn't enough - he's looking to become the first person ever to do it backwards.
Keith Clarke (41) is no stranger to feats of endurance and his efforts have already raised well over £100,000 for cancer charities and been awarded the British Empire Medal for his fundraising feats.
The personal trainer has already completed two marathons by running backwards, climbed Slieve Donard 12 times in a row - which took 37 hours - trekked up Slieve Gullion and Slieve Gallion in the Sperrins 20 times in a row and along with two friends completed a Decaman event in 2012 which raised £25k for Cancer Focus NI - that's a 2.4-mile swim, followed by a 112-mile bike and then a marathon (26.2miles) each day for 10 days.
Now he's looking to top all his previous achievements on December 15, and well into December 16, by taking on his almost 40-hour challenge at Eskragh Lough in his home town of Dungannon to raise money for Cancer Focus NI.
"I've always tried to do something a bit different, a bit whacky," he said. "My grandfather passed away from cancer. But so many other people are affected by this devastating disease that I am delighted to raise funds for such a worthwhile cause.
"Cancer Focus NI is such a good charity and does amazing work with people who have cancer and helping to support their families. I'll have people with me on the run and I've been training out there so you could say I know the route backwards.
"There's a track I'll be following and I'll have to do it 400 times to get the 100 miles up.
"Running backwards does put different strains on your body. Hamstrings take a battering, the neck muscles feel the strain and my feet swell up a lot more than when I'm running forwards.
"But I'm ready for the challenge. There is no record of anyone having done this before and it'll be tough - but not as tough as going through cancer."
The gruelling challenge will start at Eskragh Lough at 6am on Saturday, December 15. Supporters are welcome to drop by and give him a cheer along with his wife Cynthia and children Lance (9) and Madison (7).
"My kids are fitness mad and they'll be joining me for about 10k of the run," said Keith.
"But the invitation is there for anyone who'd like to come along and lend their support. There'll be a great buzz.
"I'm expecting to complete the 100 miles in 35-40 hours, so it'll be well into Sunday evening before I finish, but it should be worth the effort in the end."
Cancer Focus NI fundraiser Emma McCann said: "We are very lucky Keith has chosen to support us and we're very grateful to the local community who gets behind him too."
Anyone who would like to support Keith can visit www.justgiving.com and search Keith-Clarke100