Hot new chapter in the adventures of Rosie

When your grandmother pushes you out of the house at 12 years old with a packet of biscuits and a pony, telling you to come back a couple of days later when you've got something interesting to write about, it is probable you will grow up having a slightly unusual view of life.

Character-forming episodes like this have formed a large part of Rosie Swale's life. She has sailed with a young family to Australia and back in a 30ft catamaran, rounding Cape Horn on the way; crossed the Atlantic single-handed in a 17ft plywood cutter that was found in a cowshed; travelled the length of Chile (3,500 miles) with two horses and walked alone across Albania (which by all accounts is like 13th century feudal England).

Is it any wonder that along the way there has been some less than savoury publicity (sharing a home with a sex-change merchant seaman, for example)? And is it any wonder that at 50, she is eschewing middle-aged pursuits like jam-making, taking fuchsia cuttings and collecting glass animals, and instead planning to run in the world's toughest race?

The Marathon des Sables lasts six days and covers 240 kilometres of the Sahara Desert in temperatures averaging 120F. Chris Lawrence, who organises the British end of the race, says: "We took the temperature in a sand dune last year and it was 61C [142F]."

It's not just the heat. The terrain varies between rock and sand, though it's mostly lots of the latter, with plenty of dunes 200ft high. Running on desert sand is described as "like running through spiteful treacle". It gets everywhere, grates away at your skin and turns your feet and shins into huge blisters.

The shortest stretch is 18km. But on the fourth day, runners have to cover nearly 80km, and finish in the dark. They must carry all their personal belongings, including food, cooking utensils and tents in a rucksack with a 15kg weight limit. Not too much room for three-course meals there. The organisers generously supply water - but there's a limit to how much you are allowed. Throw in sundry hazards such as snakes and scorpions, and you start to see why the race, which starts on 5 April, is billed as being tougher than an SAS entrance exam.

What is less obvious is why about 350 people, with 40 from the UK, are willing to pay pounds 1,900 for such suffering and the chance to win a mere Fr30,000 (about pounds 3,250). Lawrence, who runs a travel agency called The Best of Morocco, says: "You have to be potentially certifiable to do this. Winning is incidental. It's for someone who wants to do something a little crazy." Like the woman who entered in a floral dress and high-heeled shoes. She lasted a day."

Swale, however, is well aware of the hazards. She has been running 70 miles a week along the beach and cliff paths around Tenby, where she lives in a small flat with her photographer husband, Chris. "I haven't been able to get any training in high temperatures but I'm used to running with the rucksack and I feel ready for the race. I want to get round - and not as a hobbler."

There seems little chance of that. It's easy to see why Swale says: "I have never liked just to arrive somewhere." The influences of her extraordinary childhood still show in her approach to life. Born in Switzerland, she was adopted because her mother couldn't look after her. The wife of a village postman cared for her until her Anglo-Irish grandmother arrived out of the blue and took her back to rural Ireland. Granny had strange views on education, and little Rosie didn't go to school until she was 13. "I really only learnt maths when I had to learn astronavigation for the Atlantic crossing," she says.

But her grandmother had more enlightened views about English, even if her teaching approach was somewhat idiosyncratic. It even influenced Rosie at 10, when she sent a love story to a women's magazine and had it rejected because she didn't have enough experience.

She trained as a local newspaper reporter, and still writes occasional articles, though most of her writing now is books. She has done five, though she admits that the early ones, written in the days when she was the darling of the tabloids, are not very good. There is another in the pipeline, though the planned one on Libya came to naught when she and her husband were deported twice. "I wanted to do a traveller's view of the country but we kept being shown round tractor factories."

Walking was once her preferred mode of exploring; now it is running, which suits her restless spirit. She claims that she is happy, settled at last in her home life and needing nothing, but she can't sit still. In the quiet seafront hotel where we meet, she is up and down like a shop steward at a strike meeting.

Others in the room listen in fascination. It's not just that they are sharing a room with a striking woman decked out for a trans-Sahara run with a rucksack full of potatoes, who is telling stories about having a mountain lion in her tent in Chile. She also has a voice that would cut through a Force 8 in mid-Atlantic. She is very popular on the lecture circuit, probably because they save on hiring a microphone.

It's impossible not to like her. She has the enthusiasm of a puppy, whether talking about her family (horribly normal) or the charity she is running for - SPANA, a veterinary charity that recognises animals as an essential part of North African and Middle Eastern families' lives.

She is approaching the Sahara race (still open for entries if you're not doing anything in April) with the excitement of a child on Christmas Eve. "When I sailed round the world, I thought that was it. Now I realise it was only the beginning." And granny would certainly have approved.