The Countess of Wessex said she was overwhelmed as she pedalled through the gates of Buckingham Palace after completing a 450-mile cycling challenge from Edinburgh to London.
Sophie and her team set out from the Palace of Holyroodhouse in Scotland on Monday for the ride in support of the Duke of Edinburgh's Award to celebrate the scheme's 60th anniversary.
The Countess was greeted by crowds of supporters and the Earl of Wessex, who presented her with a Diamond Pin to mark her completion of the Diamond Challenge.
Her children Lady Louise Windsor, 12, and eight-year-old James, Viscount Severn, as well as her father, Christopher Rhys-Jones, congratulated and hugged the Countess in the forecourt of Buckingham Palace on Sunday afternoon.
Sophie, clad in black lycra shorts and an aqua cycling top, told the Press Association: "If you told me that I would feel like this at the end of seven days cycling I would never have believed you.
"I do feel good and it has been a lovely, lovely experience.
"We have been extraordinarily lucky with the weather and I do think that if it had been driving wind and rain you would have seen a slightly shabbier, slightly more dishevelled and tired looking party arriving."
The 51-year-old royal set off on the final leg from RAF Halton in Aylesbury, Buckinghamshire, on Sunday morning to pedal more than 40 miles into central London.
She cycled down The Mall on her matching black and aqua blue road bike past jubilant crowds who clapped as she pedalled through the Palace gates.
But a beaming Sophie said she was not in pain and would conjure up another challenge.
"I expected there to be parts that I wouldn't have wanted to think about right now but actually no, I'm absolutely fine."
While she plans to take it easy for the next couple of days, the Countess said: "I feel like I don't want to lose what I've done so I've got to get out there and do something else."
On the toughest days, Sophie cycled more than 80 miles and spent at least five hours on the saddle.
"The hardest parts are the hills but you just have to select the right gear which doesn't always happen in the way that you hope it will."
She said that the final mile along the Mall was "really special" and that it was "overwhelming" to see so many people.
Sophie was 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 included members from the Army Physical Training Corps and Boardman bikes.
The Countess, whose only prior experience of cycling was with her children and "never for more than the odd hour", raised more than £180,000 for the Duke of Edinburgh Award.
She wrote on her JustGiving fundraising page that she decided to take on the challenge "to help change the lives of young people".
Sophie said she had always found cycling to be "one of the more testing forms of exercise physically and mentally" but wanted to do something outside her comfort zone to raise money for the award.
"The DofE sometimes struggles to gain the attention it deserves. I hope by doing this bike ride I will not only help them achieve their Diamond Challenge Fund goals, but may also interest others in doing their own challenges, no matter how big or small."