LADY Caroline Lamb famously said of her lover that he was "mad, bad and dangerous to know". The man in question was Lord Byron.

He was the most glamourous of the Romantic poets who, in between writing and womanising, found time to serve as a leader with Italy's revolutionary Carbonari in its struggle against Austria, swim the Hellespont and fight against the Turks in the Greek War of Independence. All of these adventures he squeezed into 36 remarkable years.

It's the kind of action-packed life that Sir Ranulph Fiennes might relate to and it's perhaps no coincidence that he has borrowed Lady Lamb's memorable remark for the title of his autobiography. He has been described by the Guinness Book of Records as "the world's greatest living explorer" but Fiennes, or "Ran" as he prefers to be called, views his extraordinary achievements in more pragmatic terms.

"I didn't set out to lead expeditions or anything dramatic like that. My aim was to command the Royal Scots Greys, like my father," he says.

His story, though, is anything but dull. This is a man who has dragged a sledge bearing the equivalent weight of three men across polar ice in temperatures averaging –40C, completed seven marathons on seven continents in seven successive days and cut off his own frostbitten fingers with a saw.

Fiennes was born in 1944 into a semi-aristocratic family that arrived in Britain at the time of William the Conqueror. He never met his father, Lieutenant Colonel Sir Ranulph Twisleton-Wykeham-Fiennes, 2nd Baronet, who was killed after stepping on a German mine at Monte Cassino in 1943.

After the war, his mother moved the family to South Africa, where Ranulph remained until he was 12, before returning to be educated at Eton.

From an early age he wanted to follow in his father's footsteps, but this required him passing A-level maths and physics in order to go to Sandhurst – something, he says, he was "badly designed" for.

He did join the British Army, though, and was seconded into the SAS and promptly booted out, following what he calls "that Castle Combe business". He's referring to an incident that led to him being fined £500 for his part in a plot to blow up the set of Dr Dolittle, which was being filmed in the picturesque Wiltshire village, much to the irritation of some locals.

After spending a couple of years serving with the army of the Sultan of Oman, for which he received the country's Bravery Medal while on active service, he found himself on civvy street with little or no job prospects.

During his stint in the army he had been an adventure training officer and surmised that leading overseas expeditions might be a way of making a living. By this time he had married his childhood sweetheart, Ginny, who became both his confidante and emotional support.

"We quickly found out that if you want to go on expeditions then you need money, and we didn't have any. So we had a rule that everything must be sponsored, from the tents down to the drawing pins," he says.

"The trouble with getting sponsors is they want to know the expedition will be covered by the top media and the trouble with that is they are only interested in covering something that hasn't been done before.

"This is difficult, because you and your team have to do better than those who went before you. So when they had to quit because of frostbite you have to make sure you don't get frostbite. Rather than setting out to endure it, you set out to avoid it."

Fiennes has led more than 30 expeditions: travelling up the White Nile in a hovercraft; parachuting on to Europe's highest glacier in spite of his vertigo; and forcing his way up 4,000 miles of the mightiest rivers Canada and Alaska have to offer.

He also became the first man to reach both poles by land, and the first person to cross the Antarctic unsupported.

These are remarkable feats of endurance, but any attempt to fathom what makes him repeatedly put his body on the line is met with a well-practised, if polite, dismissal. TV psychiatrist Dr Anthony Clare once described an attempt to analyse him as "stirring a void with a teaspoon" and it's easy to see why.

"I'm not introspective and I'm not philosophical, although I'm sure Freud could have come up with hypotheses about it," he says.

The adventurer has had his failures. Two years ago he was within spitting distance of the top of Everest when ill-health forced him to abandon the expedition. And in 2000, his attempted solo walk to the North Pole almost cost him his life. When the sledge carrying all his food and equipment fell through weak ice, Fiennes was forced to pull them out by hand. He suffered severe frostbite to the tips of the fingers on his left hand, forcing him to abandon the attempt.

On returning home, his surgeon told him he would have to wait five months while the partially damaged tissue healed before they could be amputated. After being told the pre-operation costs were £6,000 he took matters into his own hands.

"The finger tips were totally mummified, and when you touched anything it was agony," he says. "I wasn't prepared to endure four more months of pain, when all I had to do was go down to my shed, take out a Black and Decker and cut off the dead bits. It only took a couple of days and I did a decent job."

If that sounds astonishing, then what happened in 2003 almost defies belief. Fiennes and his friend Dr Mike Stroud agreed to attempt the extraordinary feat of completing seven marathons in seven days on seven continents to raise money for the British Heart Foundation.

While waiting for a plane to take off at Bristol Airport he suffered a heart attack which required a double bypass operation and left him in a coma for three days. Despite his brush with death he was determined to carry on, even though he only had two-and-a-half months to prepare.

"It was all arranged, with the schedule and the sponsors, so it couldn't be put off." Fiennes sought his surgeon's advice. "He told me he had performed 3,000 similar operations, and no one had asked if they could run a marathon. He said that I should not let my heart rate exceed 130 beats a minute and, as far as I'm aware, it didn't," he says.

"The original plan was to get a day back through the international date line, but instead we ended up doing two marathons in one day. We did the London one in the morning and flew to Cairo and did the African one that evening."

After completing the fourth, in Singapore, he contemplated throwing in the towel.

"I was completely exhausted and suffering from dehydration and I thought 'I can't carry on', but then someone gave me a cup of tea and I felt better." He rallied, and the pair completed the final marathon in New York.

Much has changed in Fiennes' personal life. His adored wife Ginny died of cancer in 2004, but he has since remarried and become a father for the first time. Earlier this year, at the age of 63, he successfully climbed the notorious north face of the Eiger, raising money for the Marie Curie charity.

"There's the politically correct brigade who will tut and say I'm irresponsible because I have a daughter to think about. But I just equate her with my own experience. My dad died before I was born but I had a wonderful mother and I genuinely never missed the father-figure."

The adventurer says he isn't immune to fear. "I've found human beings far more frightening than nature or God throwing darts. It's more terrifying being alone at a train station with a load of yobbos coming your way, than it is lying in a tent listening to wolves howling in the polar night."

Fiennes is now contemplating a return to Everest. The idea of retirement is alien to him. "I hope I never have to resort to any horrors such as golf or fishing," he says.

While a lifetime exploring the wild frontiers of the natural world has brought him close to death, it has also enabled him to raise millions of pounds for charity, brought him worldwide fame and an OBE.