Sophie, Countess of Wessex's family thought she was "quite mad" when she told them she would be cycling from Scotland to London.
The 51-year-old royal is set to cycle the 450 miles from Holyrood Palace in Edinburgh to London's Buckingham Palace in support of the 60th anniversary of the Duke of Edinburgh's Award.
She shared: "At the start of this year I had almost no experience as a road cyclist. Like many people, I had only ever gone on occasional bike rides with my family.
"When I told my family what I was planning they definitely thought I was quite mad and I'm not sure they took my plan seriously at first.
"But from the start I was determined to succeed, and to be one of the many committed people who will take on a Diamond Challenge this year in support of the 60th anniversary of the Duke of Edinburgh's Award (DofE)."
And Sophie, who is married to Queen Elizabeth and Prince Philip's youngest son Prince Edward, praised Sandhurst trainer Rachel McKenzie for helping get her into shape.
She added: "If she [Rachel] was daunted by the task of getting me fit enough for the challenge, she didn't show it. It was clear we had some serious work to do.
"Those early days were pure hell. Every incline was like the north face of the Eiger and I prayed for the end of every ride! I confess I even shed a tear or two of desperation and frustration."
Meanwhile, Sophie also shared her concerns about being able to achieve the required fitness at her age.
Writing for MailOnline, she added: "I turned 50 last year and there have been moments when I questioned if I would be able to achieve the fitness required. But the more time passed, the more I believed in myself.
"Six months on I am fitter than ever. I have been surprised by how much I enjoy cycling, which I aspire to continue once my Challenge is complete."