The 60-year-old who was given the "greatest" designation by the Guinness Book of World Records for his international exploits is engaged to a women 22 years his junior, and plans to honeymoon on Mount Everest to overcome a lifelong fear of heights, even though it is less than two years since he suffered a major heart attack.
Last February while he was running seven marathons in seven days, his wife of 34 years, Ginny Fiennes died of stomach cancer.
His bride-to-be is 38-year-old horse trainer Louise Millington, whom The Times of London said has moved in with him at his cattle farm near Exford.
The wedding is planned for March, with bride and groom departing promptly afterwards for Tibet.
Fiennes will attempt the world's highest peak in May as part of a guided expedition, hoping to raise up to $3.5 million for the British Heart Foundation.