There are some things Anna Wardley knows about her plan to swim around Tiree, mainly that it will be cold and the tide will be strong.

There are also some things she doesn't know, mainly what the weather will be like and whether she will complete it (if she does, she will be the first swimmer ever to do so). And there is one thing she knows about herself: she is not scared.

That isn't to say 36-year-old Wardley is taking the swim around Tiree for granted. She knows the 30-mile route round the Hebridean island will be difficult, but she's been training six days a week and she's done her research. Which is why she's not scared. Respectful, yes; scared, no.

"No one has done this swim before," she says, "and there's probably a good reason for that. The water's cold, the tides are strong and it's a case of timing it and making the corners at the right points or you'll get swept out to sea. A lot of things have to fall into place but I'm confident."

And Tiree is only part of it. Wardley has set herself the challenge of swimming 150 miles around five islands: Tiree, the Isle of Wight, Jersey, Portsea in Hampshire and Dragonera in Spain. She is doing it to raise money for charity but what's really extraordinary is that Wardley used to be a twice-a-week swimmer who couldn't do the crawl. Now she is one of the heroines of wild swimming, a movement that's not only growing in popularity but has women at the forefront.

"Open water swimming is one of the few sports where women are better generally than men," says Wardley, "and it's to do with where we carry our subcutaneous fat – we have more body fat on the surface which means we can withstand the cold better. Whenever it comes down to endurance rather than speed and in colder water, women tend to be better."

This seems to be borne out when you look at the achievements in open water swimming. The record for crossing the Channel the most times, for example, is held by a woman, Alison Streeter, who has completed the swim 43 times.

Wardley has also won recognition and respect for her achievements including the World Ice Swimming Championships in Russia but it's the kind of extreme activity that can put a big strain on the body, as Professor Mike Tipton of the Extreme Environments Laboratory in Portsmouth, confirms. "Cold water immersion is one of the greatest stresses the body can be exposed to," he says. "It is one that could not be undertaken without years of preparation."

So why does Wardley do it? Part of the reason is to raise money for her charities, which include The Samaritans as a mark of respect to her dad, who committed suicide when she was nine. But another part of it is the Edmund Hillary thing: because it's there. "I guess it's the goal," says Wardley. "And in order to achieve the goals, there is no middle ground."

So far the training is going well. Six days a week, Wardley is up at 6am and in the sea near her home in Gosport, Hampshire, on the south coast, or at the gym. Like the equipment she needs such as support vessels, it's all paid for by sponsors.

And now here she is: just three weeks from the first swim in Spain on May 27. After the third swim around Jersey in July, she will be taking time off to work for the London Olympic organising committee before training resumes for Tiree. By the end of the challenge, Wardley hopes to have raised £50,000, but the woman who until recently could only do the breaststroke is hoping for something else as well. "I want to show people what you can achieve when you put the cliché 'anything is possible' to the test," she says.

For more information, www.annawardley.com