"That equates to two marathons a day for eight days," says Moody, who made 223 appearances and won two Heineken Cups and seven Premiership titles during a glittering career at Welford Road.
Moody is taking on the challenge, which starts on February 3, to raise money for Hope HIV and will be trekking with the charity's founder Phil Wall and polar explorer Alan Chambers.
Moody, who won back-to-back Heineken Cups with Tigers in 2001 and 2002 and was part of the England squad that won the World Cup in 2003, says it is his biggest challenge to date.
"It is, because of the difficulty of the race itself, the conditions we face and the fact that I suffer from ulcerative colitis, an inflammatory bowel condition," says Moody, who played 71 times for his country and won three caps for the British & Irish Lions.
"Doing something like this with that condition, where you can't be too far away from the toilet at any given time, makes it tougher. It's about proving to people that suffer from the same illnesses, that you can still have an exciting and full life, and raising money for a good cause in Hope HIV.
"As a sportsman, you spend your whole career challenging yourself physically and mentally every moment of your day, whether that be in the gym, on the training pitch or in a match. When I retired I really missed that and this is a massive challenge, physically and mentally."
The challenge facing Moody and his team is underlined by the fact an average of 75 contestants start the race each year and less than half finish it.
"It's not about winning," he says. "It's getting to the end and completing it in the eight days - that's our goal."
To sponsor Lewis Moody on his Yukon Arctic Ultra challenge please either text MYGC99 with a donation of £1, £5 or £10 to 70070 or go to www.justgiving.com/teams/mygreatestchallenge. For more information and for updates visit www.mygreatestchallenge.org