Just imagine standing in front of a fire hose, ice cold, relentless spray, sharp as nails, unable to feel your hands and feet, enduring relentless pounding as your boat scales and slams off roller-coaster-sized waves… ”

This is how the Velux 5 Oceans – the longest, toughest and most dangerous single-handed round-the-world yacht race – is described on the organiser’s website. And these are the conditions Sir Robin Knox-Johnston will soon be confronting at a time when the greatest challenge faced by many of his contemporaries is shuffling down to the post office every week to collect a pension.

At 67, by far the oldest competitor in the race, Knox-Johnston is due to set sail from Bilbao in Spain next Sunday with seven other intrepid solo sailors on the adventure of a lifetime. The first leg is nonstop round the Cape of Good Hope to Fremantle, Western Australia; the second round Cape Horn to Virginia in the United States; and the last across the Atlantic back to Bilbao. He will be 68 by the time the race finishes next April, after a 30,000-mile odyssey through some of the most hostile seas on Earth.

Knox-Johnston is, of course, assured of a place in history. In 1969 he became the first man to sail nonstop around the world – a feat many people believed at that time to be impossible. The fact that he did it in a little wooden ketch, only 32ft long, added further lustre to his achievement. In the Velux 5 Oceans race, he will be at the helm of an Open 60, a formidable, demanding, state-of-the-art racing machine, stripped of all creature comforts to save weight. He will sleep in a narrow canvas sling called a pipe cot, cook on a single methylated-spirits burner and use a bucket to deal with bodily functions. Sailing an Open 60 alone, 24 hours a day, for weeks on end, in the storm-prone southern oceans is not for the faint-hearted, and certainly not to be recommended for those who have reached pensionable age. So why in heaven’s name is he doing it?

Sitting over a mug of tea in the kitchen of his cottage in Devon, he draws deeply on a Marlboro Light and shrugs in a self-deprecating, search-me fashion.

“Just because you pass the age of 65 doesn’t mean your brain turns to porridge and you forget everything you know. I think retirement at 65 is bloody nonsense. I know I am not as agile as I used to be and don’t have the same muscular power, but I’m still pretty wiry and quite fit.

I think I’ve got one last race in me, and this is the one. I started thinking about it 10 months ago, but didn’t really make a decision until my offer for the boat in which I will be competing was accepted. I had two feelings then: ‘Crikey! What have I done?’ And then ‘Yes! I want to do it.’”

Round-the-world yacht racing is a very expensive sport and Knox-Johnston was obliged to take out a substantial bank loan to buy his nine-year-old boat, formerly owned by the Italian yachtsman Giovanni Soldini, who won the race in 1999. He renamed her Grey Power in a nod to his advancing years. It is now called, rather unromantically, Saga Insurance, in deference to the sponsor he had to struggle hard to find. He estimates that he will have spent around £500,000 by the time the race starts.

To qualify, he was required to sail her at least 2,000 miles, solo, between ports, which he did in July, crossing the North Atlantic from Halifax in Nova Scotia to Falmouth in Cornwall in just 14 days. It was his first single-handed passage for more than 15 years but was, he says breezily, “no trouble at all”. He only got 40 minutes’ sleep in the final 2½ days but felt he coped perfectly well. Ever since then he has spent every spare minute preparing his boat for the race, doing much of the work himself to save money. Knox-Johnston is not only the oldest competitor, he is also the least well prepared, entering at virtually the last minute, and the least well financed. None of this bothers him in the slightest. “I really needed this project,” he says suddenly, lighting another cigarette and staring out into the garden, lost for a moment in his own thoughts.

Master mariner, knight of the realm, fearless adventurer, a sailor through and through, Knox-Johnston does not appear at first glance to be the kind of man to wear his heart on his sleeve.

With his piercing eyes, weather-beaten face, tight curly grey hair and grizzly beard he looks every inch a horny-handed old sea dog. But behind the gruff, no-nonsense demeanour, there is a sadness he cannot conceal for very long. It is as if a shadow has fallen across his life.

Three years ago he lost his wife, Suzanne, the great love of his life, to ovarian cancer, and he candidly admits that he is only now recovering from the shock of her death. Throwing himself into the hectic preparations for a round-the-world race has undoubtedly helped. Significantly, he says he probably would not have entered the race had she still been alive.

The Knox-Johnston love story reads like something out of a Mills & Boon novel. He met his future wife when he was eight and she was six; she lived across the road in Beckenham, Kent, where Robin lived with his parents and four brothers. Apart from the fact that his father worked for a shipping company, there was no seafaring tradition in the Knox-Johnston family, but very early on in life Robin determined to join the Royal Navy.