It is early morning on the outskirts of Tokyo and Mara Yamauchi is three days into a routine she will put to use in a fortnight: bed by six, awake by three, breakfast by four, running by five. She is not alone. “You will be surprised by the amount of people who are out at that time,” she says. “Some are exercising, others are cycling, some are just walking. By six, people are on their way to work. It is the only way to do things in the summer because it is so hot.”

On Saturday, the 11th world athletics championships begin three hours away in Osaka, and such will be the conditions when Yamauchi takes to the start-line of the marathon on the final Sunday that it is expected to be 26C at 7am. As the runners approach the finish, the level of humidity will make breathing difficult. It is no wonder she is sleeping long before the sun sets to ensure her body is ready for such an early embrace in an event that could shape her destiny.

But her ability to slip into such a routine has arrived on the back of jet-lag gained from preparing like many members of the Japanese team for Osaka: by being as far away from the country as possible. Yamauchi should know. Oxford-born and Oxford University-educated, she represents Great Britain but her home is Japan, the epicentre of women’s marathon running. Yet she has just returned from a month of training in, of all places, the exclusive Swiss ski resort of St Moritz.

“The main reason is that the summer is too hot to train here,” says Yamauchi, 34. “It is very humid and while the advantage is that you become used to running in such weather, there is the danger you will become too exhausted. There are altitude places in Japan, but St Moritz is much better. It is 1,800m [above sea level], they have a good track, the valley is flat, the surfaces are generally off-road and in terms of variety, it is pretty much the best place for training I have been to. There are cooler places in the north of Japan and some people train in China, but even in St Moritz, two of the Japanese athletes were there at the same time as us.”

When the 55 other members of the British team arrive in Osaka on Thursday from their base in Macau, China, Yamauchi’s best advice for them is not to “do anything crazy” like spend too much time in the sun. “They will need plenty of rest,” she says. She will, of course, tell them that in English; though should they like to know the Japanese version, she can do that too. In the absence of world record-holder Paula Radcliffe, who has not run since the birth of her daughter Isla in January, Yamauchi is Britain’s leading woman marathoner. She was sixth in London this year and since her 18th-place finish at the last world championships in Helsinki in 2005 – which Radcliffe won – she has brought her time down from a modest 2hr 31min 26sec to a challenging 2hr 25min 13sec.

While most of the attention in Japan will be on their nationals, such as Yumiko Hara, fourth fastest in the world this year, the winner of the Osaka women’s marathon in January and one of the favourites for gold, home televi-sion cameras are likely to focus on the adopted Briton, too.

Formerly Mara Myers of Harrow AC, Mara Yamauchi, still of Harrow AC in London, has spent six years in Japan. The first four of those was while working for the British embassy, where one assignment was interpreting for former prime minister Baroness Thatcher, and another saw the conversation turn to Blackburn Rovers with the then foreign secretary Jack Straw. During her first period in Tokyo, she met Shigetoshi Yamauchi, whom she married before work took them back to England. He trained as a lawyer before being assigned to a job in Tokyo. At the start of last year, she took unpaid leave from the embassy until after the Olympic Games in Beijing next year to ensure she could have the best possible preparation and he is now not working, but helping her. They could have returned to Britain, but the lure of Tokyo was too strong. “Mara-thon running is the most popular athletics event in the country,” she says.

And one of the most successful. It has provided the last two women’s Olympic champions, Naoko Takahashi in Sydney in 2000, and Mizuki Noguchi in Athens in 2004. The latter will not be in Osaka as she prepares to defend her title in Beijing, by which time Yamauchi hopes to have made even greater strides. She has not long been a member of the Japanese athletics club Second Wind, where she runs with Kiyoko Shimahara, a member of the Japanese team for the race in Osaka. “Since we have been living here and I have been racing here, I have become a little bit known on the Japanese racing circuit,” says Yamauchi. “There is plenty of expectation here. A lot of the club members will be going to Osaka to support us. I have a Japanese surname, so people identify with that, even though I am not Japanese. I understand the language, I know how to get around, which will make the whole experience easier for me than someone coming from overseas, because Japan is different from the UK.”

Her familiarity with the course is growing by the day. She used the January marathon as a training run, stopping 7km from the finish but content that she had covered all the ground she needed.

“It is a fast course, it is pretty flat, there are two little hilly bits but I feel I know it,” she says. “It is an out-and-back course, so I ran that remaining 7km at the start. I have been watching videos of the race.”

Yet, it seems, everything will go back to the heat.

“I have some worries about coping because the conditions are so extreme,” she says. “But I have done as much as I can to prepare in the best way possible. Since Helsinki, I have learnt to cope with injuries, I have learnt about the limits of what I can do and I have not pushed myself beyond that.

“I want to be top eight, but because of the weather conditions it will make it an open race. Anyone who can run 2:30 could win.”