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Mr Spearing (63) flies out on September 2 to hike from St Bees to Robin Hood Bay, a distance he estimated at 320km.
Several years ago, he walked the length of Great Britain, from south to north - Lands End to John o'Groats - which took him 75 days to complete.
He figured he could now traverse it the other way and he expected it would take between 10 and 12 days.
Mr Spearing, who retired to Oamaru about six years ago after 27 years in the fire service in Auckland, walked from Mexico to Canada in 1991.
He read a book about a British man who had done the same trip and thought he would give it a shot.
Describing it as a "real milestone" in his life, Mr Spearing said his memories of the epic journey were still vivid.
It took five months to complete the trip, which was entirely on foot and off-road through desert areas in Mexico, into the Sierra Nevada range in California, the Cascade ranges of Oregon and Washington, before he "popped out" in the Okanogan Forest in British Columbia, Canada.
"I was kind of really tempted to keep going because it became a way of life," Mr Spearing said.
He wrote about his adventures in a book which was published in the United States.
It was serialised on National Radio earlier this year.
While the prospect of bears was always a worry on the United States trip - "they got my food one night" - it was dogs that were the problem in Great Britain.
On his last traverse, he seemed to be a "dog magnet", with dogs going "berserk" on isolated properties.
That trip was not as enjoyable as his US adventure.
"Britain's too populated for a start. In America, you really were in the wilderness."
Mr Spearing walked the length of the North Island in 1996-97, which he said was not particularly enjoyable.
"With America, everything was new; all the wildlife was different, everything was exciting.
"I know what New Zealand's like. It was just a slog really," he said.
He once cycled across the Nullarbor, from Kalgoorlie to Port Augusta, in Australia, with another fireman, a trip that took two and a-half weeks and was much more interesting than expected.
Mr Spearing did have moments, during his trips, when he questioned what he was doing, but knew he had to keep going.
He was unsure what his next adventure would be.
"All of a sudden, I'll think of something. You've got to have a mission in life."
• Details about his book and US trip can be found on www.danceswithmarmots.com