Despite suffering from a heart attack and undergoing a double heart bypass operation just four months before, Fiennes joined Stroud again in 2003 to complete seven marathons in seven days on seven continents in the Land Rover 7x7x7 Challenge for the British Heart Foundation. "In retrospect I wouldn't have done it. I wouldn't do it again. It was Mike Stroud's idea". Their series of marathons was as follows:
Fiennes' career as an author has developed alongside his career as an explorer: he is the author of 24 fiction and non-fiction books, including The Feather Men. In 2003, he published a biography of Captain Robert Falcon Scott which attempted to provide a robust defence of Scott's achievements and reputation, which had been strongly questioned by biographers such as Roland Huntford. Although others have made comparisons between Fiennes and Scott, Fiennes says he identifies more with Lawrence Oates, another member of Scott's doomed Antarctic team.
In 2003, while attempting to scale Mount Everest Fiennes suffered a heart attack and later underwent emergency bypass surgery.