Sophie and her team are pedalling from the Palace of Holyroodhouse in Edinburgh to Buckingham Palace in London in support of the Duke of Edinburgh’s Award as the scheme celebrates its 60th anniversary.
Philip himself and her husband Prince Edward, the Earl of Wessex, waved her off on the Diamond Challenge on a bright and crisp autumnal morning.
Speaking minutes before she got going, Sophie said the route ahead of her is a daunting prospect and added: “Standing here right now, I am very nervous. The prospect of the 450 miles is pretty daunting.
“I’ve been very fortunate to be supported by a lot of family and friends, and I’m incredibly grateful to everybody who’s actually supported me. I’m hugely grateful.”
The Countess was dressed for the part in a blue cycling top, black shorts and white helmet.
She has pedalled almost 3,000 miles, including cycling on her static bike at Balmoral, Aberdeenshire, in preparation for the charity challenge.
Rachel McKenzie, a physical training instructor (PTI) at the Royal Military Academy Sandhurst, has been putting the royal through her paces and is joining her on the trip, which will take about seven days to complete.
Sophie told broadcasters: “I’ve been very lucky that a PTI from Sandhurst has been pushing me around quite a lot of hills and lanes of Hampshire, and all sorts of places actually.
“It’s been a lot of hard work, a lot of early mornings and many hours in the saddle.”
The Countess also told how she got into watching cycling at the time of the 2002 Commonwealth Games in Manchester, and later the London 2012 Olympics.
“They were all so inspirational and exciting,” she said.
“But I never in my wildest dreams thought that I would then end up by doing this.
“It wasn’t something that made me want to get on to the bike. It was only when I was deciding what challenge I was going to do that I ended up choosing cycling, but it was more by accident than design.”
Sophie is joined by six team members including riders drawn from the four Royal regiments she is linked to - 5th Battalion The Rifles, RAF Wittering, Queen Alexandra’s Royal Army Nursing Corps and Corps of Army Music. The team also includes members from the Army Physical Training Corps and Boardman bikes.
Before the ride set off, Philip and Edward met young people who have completed their own Diamond Challenges in the award scheme’s 60th year.
Philip, Sophie’s father-in-law, set up the award programme in 1956, inspired by his headmaster Kurt Hahn and his school days at Gordonstoun, Moray.
It has become one of the best known self-development and adventure schemes for 14 to 24-year-olds, with around 2.5 million awards achieved in the UK since its inception.