The chartered surveyor and digital project manager plan to run, swim, cycle and row the 3,000 miles from Marble Arch to the island of Svalbard.
Having never rowed before, and with only basic skills in swimming, they have been secretly training constantly for the past year – not wanting anyone to talk them out of it.
The 33-year-olds hope to raise £100,000 for the Scout Association.
Rob, from Bristol, said: “As a former Cub Scout myself, I know the amazing benefits that Scouting offers. It set me on a path to adventure and gave me skills and values I have to this day.
“It’s about giving every chance to every child. We want all young people, especially those in the most deprived parts of the UK, to get the chance to develop skills for life.”
Poldy, who grew up in rural Argyll, agrees: “I just think it is really important that we encourage children.”
Beginning on Friday, the pair will run to Dover – the equivalent of three marathons in three days – then swim for around 14 hours to Calais where they will pick up their bikes.
They will cycle more than 2,000 miles through seven countries from France to the very north of Norway, before rowing from there to Svalbard.
The row will take them through the icy waters of three different seas, the Norwegian, Barents and Greenland Sea for a total of 700 miles.
Poldy added: “[The boat] is so difficult to manoeuvre, it is so big and heavy. But we are confident.”
Chief Scout Bear Grylls described the mission as: “An incredible expedition that will test the team to the very limits. I so admire their spirit and courage – and raising funds for the Scouts is such a powerful way to change young lives.”
To donate go to arch2arctic.com