The Edinburgh-based athlete said the Scottish section of the 15-day epic was “just brutal”, with “unrelenting” rain and his hands left feeling like blocks of ice.
In a new film about the 2017 epic, Around Britain, released on Friday by GCN+, Beaumont admitted it had made him question whether he was up for riding across the world.
He said: “I was sitting at John O’Groats and it was lovely being back, but I thought ‘crikey, if this was the world and I’m less than two weeks in, how am I going to do two-and-a-half months of this?’”
Beaumont, 39, said on Thursday: “I don’t think it proved we could do the world – it scared the hell out of me.”
However, five months later he triumphantly sliced one third off the 123 days round-the-world record by completing it in 79 days.
Reflecting in the film about the Welsh leg of the Around Britain ride, Beaumont said: “I was recovering from the south west – I’d found Devon and Cornwall pretty punishing, that north coast in particular had taken it out on my legs.
"Little did I know that by the time we got to Scotland, it would be much, much worse.”
He said on reaching Scotland: “The weather came into its own.
"I will always remember the ride up towards Oban.
"I was in a miserable place – I remember my hands were blocks of ice. I was so cold and it was lashing it down with rain.”
However, Beaumont said he had been cheered by supporters he encountered along the way, including the future Paralympian cyclist Fin Graham, then 17.
He said: "This guy rode across Skye with me, over the Skye bridge, up through Plockton, heading towards Shieldaig.
"He stayed with me until it got dark and must have done the best part of 100 miles.
"He lifted my spirits, he gave me a sense of perspective.
"He was just such a lovely lad and he was as strong as an ox.
"He’s now Team GB. At that time, nobody knew who he was.”
Beaumont is now turning his attention to more “Explore Your Boundaries” rides following Scottish council boundaries, inspired by Covid lockdown travel restrictions, with three days of filming in Argyll next month with Inverness cyclist Jenny Graham, the fastest woman to ride round the world.
He said: “I love the idea you don’t need to go far to find adventure.
"Such opportunities to find new routes will hopefully inspire people to get out.
"We are spoilt for choice in terms of places to go in Scotland.
"I want to give people the quiet confidence of getting out there and having adventures themselves, to appreciate wild spaces and get a new understanding of what’s on their doorstep.”
Meantime, in June, he will attempt to become the first British cyclist to win the Race Across America, a 3,000-mile, four-day challenge from California to Maryland, which was launched 40 years ago.