British adventurer Sir Ranulph Fiennes, who had life-saving heart surgery four months ago, has set out on his latest challenge - to run seven marathons in seven days.
Sir Ranulph has taken medical advice
He will start in Antarctica on Sunday, then fly on to Santiago, Sydney, Singapore, London, Cairo, finishing in New York on 2 November.
His efforts will raise money for the British Heart Foundation.
The 59-year-old had an emergency by-pass operation at Bristol Royal Infirmary in June.
He will be accompanied on the marathons by his long-time adventure companion Dr Mike Stroud and support vehicles will carry a defibrillator in case he has any heart problems.
A spokesman for Sir Ranulph said that despite possible concerns over his health, he had only taken out ordinary holiday insurance.
The first marathon, on King George Island, Antarctica, could set the pace for the trip if the weather turns bad.
"If you're running around on tarmac you usually go faster than you do on snow and ice, so that is going to slow up the 26 miles considerably if the weather is good," Sir Ranulph told BBC News.
"If we have a whiteout and a wind and a fog that's going to cause big, big problems because we're on a really tight schedule.
"If you take slightly too long and miss one flight then the whole thing fails."
After the cold of Antarctica, there will be two more marathons before hitting the humidity of Singapore.
"That might cause a problem because by then we'll be very debilitated and more easily exhausted than on the first marathon because of lack of time to put all the correct things back into your body," Sir Ranulph said.
"Normally one is advised to rest for three weeks to recuperate between a typical marathon in an English-type climate."
He will then fly to London and run in the morning of 31 October before jetting to Africa to begin his second marathon in a day just before midnight in Cairo.
Sir Ranulph will finish in the official New York marathon on 2 November.
"We've been advised by then that we won't be running in a straight line, so if we get in people's way we hope they'll be patient," he said.
Sir Ranulph has been awarded the OBE for "human endeavour and charitable
services", received a double clasp to his Polar Medal for outstanding
achievements in polar exploration, and holds the Sultan of Oman's Bravery
In 1982, a transglobal expedition led by Sir Ranulph was the first successful
circumnavigation of the globe on its polar axis.
In 1993, Sir Ranulph became one of the first men to walk unaided across