Sir Ranulph Fiennes looks on course to run seven marathons on seven continents in seven days after completing the fifth leg of the challenge.
Sir Ranulph Fiennes recounts his run
The adventurer crossed the finishing line at BBC Television Centre in west London at Friday lunchtime, after following the route of the 1908 Olympic marathon from Windsor, Berkshire.
He told BBC News Online, after completing the run in four hours and 41 minutes: "I feel fine. Yesterday was really terrible. But today has been great. The runners were incredible."
Asked if he would do it all again, he said: "Of course."
But he was circumspect about his chances of completing the challenge saying he was not "confident" yet.
He only arrived at Heathrow from Singapore at 0500 GMT on Friday with running mate Dr Michael Stroud, 49, and began the run at 0730 GMT.
Dr Stroud finished about an hour after Sir Ranulph, 59.
Sir Ranulph said: "Mike got really, really sick yesterday. He was urinating
blood and had diarrhoea so I couldn't use him as a pacemaker today.
"But Mike is a doctor, so what's good for him is good for me."
The two will now fly to the Egypt capital Cairo for leg six, where they begin the run at the Sphinx at midnight and race though the night.
The London run began at Windsor Castle
They then catch a 6am flight to New York where they will complete the marathon challenge.
But before the London leg, a spokeswoman for Sir Ranulph quashed speculation they were going to quit following the gruelling fourth leg in Singapore.
Sir Ranulph described the course as "hell on earth", while Dr Stroud walked much of the way after suffering a stomach upset.
The pair have so far completed runs on the Falkland Islands, Santiago in
Chile, Sydney and Singapore.
They went to the Falklands when their hopes of running on King George Island in Antarctica were dashed by bad weather.
But it was the Singapore leg, run in tropical heat and humidity, that has been the biggest challenge.
Asked afterwards if he thought he would complete the seventh marathon in New York, Sir Ranulph said: "It's getting a bit doubtful."
Sir Ranulph has fought back from a heart attack and a double heart bypass operation to carry out his latest test of endurance.
He had already promised to take on the challenge before he collapsed on an aircraft in June and had to undergo surgery.
The pair have also been running with a defibrillator to safeguard Sir Ranulph's health.
All money raised from the Land Rover 7x7x7 Challenge will be given to the British Heart Foundation. They hope to raise a six-figure sum.