Explorer Sir Ranulph Fiennes and his running partner Michael Stroud set off from the gates of Windsor Castle west of London at 7:30 a.m. local time (2:30 a.m. EST), just 2½ hours after arriving from Singapore, where they had run their fourth marathon in as many days.
Fiennes described the Singapore leg of his challenge as "hell on Earth" after temperatures soared above 90 degrees Fahrenheit.
"I couldn't stand the humidity, and also, my colleague, Mike Stroud, who's fostered me - I normally follow him, keep to his pace, but he got really, really sick," Fiennes told reporters.
And the 48-degree weather in London before Friday's 26-mile run?
"Fantastic! Cool. Even the traffic pollution here is less polluted than it was there," said the 59-year-old Fiennes, who had double bypass surgery following a heart attack in June.
"My cardiologist said as long as I don't go over 130 beats a minutes there shouldn't be any problem," he said. "My wife said to cool it, so I have been cooling it, definitely. I do what she says. I've cancelled all the competitive races."
The pair have also completed marathons in Chile, the Falkland Islands and Australia.
"Cairo's tonight, starts at the Sphinx at midnight, and then I'm going to catch a plane at 6 o'clock to New York" for Sunday's New York City Marathon, where he hopes the temperatures will also be cool.
Fiennes was once described by the Guinness Book of World Records as the "world's greatest living explorer" for leading more than 30 expeditions, including the first polar circumnavigation of the Earth, in 1982.
Friday's run follows the route of the 1908 London Olympic marathon, and takes the pair through west London.