Two former soldiers from Northampton are running from Norfolk to Wales for charity in just seven days.
Tristen Dibb and Craig Trusler, both aged 30, will attempt the 330-mile cross country run carrying food, water and medical supplies.
To complete their journey on time - which includes 10 river crossings, on of which is 20 ft deep - they will need to run an average of 45 miles a day.
"I want to do something different while making a difference," said Tristen, who works in an office in Northampton Monday to Friday. "It's going to be painful in places but it's a great challenge."
Their route from Holyhead to Lowestoft covers the length of the country running from east to west.
With no tent, they will sleep outdoors every night under a sheet of waterproof tarpaulin held up by bungee cords and will eat out of British Army ration packs.
Tristen said: "We want to everything proper army style. We're carrying everything we need with us and we've got friends at regular checkpoints along the way to resupply and to let everyone know we're safe.
"I'm dreading the river crossings, though. They can be very dangerous, so we're taking GPS trackers and will always test the current before we start swimming."
Craig served in the Royal Electrical and Mechanical Engineers, while Tristen served with the Royal Logistics Corps.
What the pair have in common, though, is they both suffered leg injuries while in the forces.
"I fell off a wall and badly fractured my leg and was medically discharged," said Tristen. "While Craig developed shin splints from running and took a voluntary discharge so he could serve again later."
Craig, a building surveyor from Northampton, said: "None of that is stopping us now, though. We run together twice a day and will spend the next five months building up our core strength.
"We're both known for taking on endurance challenges but we're never done anything this big."
Tristen has a fundraising target of £1,000 and will donate the money to Help for Heroes, while Craig wants to raise £2,000 for MacMillan Cancer Support.
"Help for Heroes are a great charity that has helped so many of my friends," said Tristen. "They do so much more than people realise and I'm so grateful for what they do."
Craig said: "I recently lost my nan to cancer and Macmillian were fantastic in their support. They were in her corner every step of the way."
The finishing line is at Celtic Gateway Bridge, in Holyhead, where the pair will be met by their friends and family to take them home.
Tristen said: "I'll be missing my little girl's birthday during the run, but I'll phone her on the day. When I get home, I think I'll stay in bed and not move for a week."
Craig said: "I'm looking forward to the adrenaline rush of crossing the finishing line and knowing I've done something amazing. It's going to be such a great experience."
Tristen's fundraising page for Help For Heroes is at www.justgiving.co.uk/tristen-dibb1.
Craig's fundraising page for MacMillan Cancer Support is at https://www.justgiving.com/fundraising/Craig-Trusler.