Despite that daunting prospect, there's no shortage of people who want to participate. Unlike the unheralded Beijing Marathon, which struggles to compete with the more glamorous races in London, New York, Boston and Berlin, the opportunity to run along China's most famous landmark attracts runners from all around the world.

'The fact that it's a sporting event that takes place on one of the wonders of the world was the main motivation for me to enter,' says Discovery Bay resident Martin Rueegg.

He will be among 1,700 runners who will converge on the Huangyaguan section of the wall, 120km from Tianjin, in the morning. About 1,000 runners will take part in 5km and 10km races. Gratton will be in this group, but his company, 2:09 Events, which is named after his best time for a marathon and organises running holidays, will be sending 50 runners from Britain to take part in the full marathon.

Most are attracted by the sheer difficulty of the race. 'There are a lot of people who've done the London and New York marathons and they're looking for a new challenge. They're bored with the standard events, so they look for tougher races, like the Himalaya 100 Mile Stage Race or the Great Wall,' says Gratton. 'For some people, the wackier the race the better it is.'

Guo Ying is one such runner. Originally from Ningbo, the 46-year-old ran her first marathon in Hong Kong in 1997. Now living in Beijing, she's run every Great Wall Marathon since 2002, except for 2003 when the race was cancelled because of Sars. But she also runs ultra-marathons such as the annual 100km Sunrise to Sunset race in Mongolia.

'I haven't been training too hard this year, but when I was really fit the Great Wall Marathon was like a training run for me,' she says.

Nevertheless, even Guo concedes that it's far from an easy race. 'If people aren't prepared properly, it's very hard because you go up and down the wall. After running 18 miles [28km], you have to climb up the steps and it's so difficult. If it's your first marathon, then it's very hard. Almost all of us have to stop and walk up the steps,' she says.

It's the 3,800 steps that are worrying Suzhou-based English teacher Sarah Shaw-Gray. The South African has never run a marathon. 'Having seen the size of the steps, I know that it will be difficult climbing up them,' says Shaw-Gray. 'But I've wanted to do a marathon for a long time and I thought if I'm going to do one, why not a tough one.'

The brainchild of a German architect named Wichard Holscher, the first Great Wall Marathon took place in 1996. It was limited initially to fewer than 100 runners but the race has steadily grown, as have other adventure events such as the Everest and Antarctica marathons.

This year, there will be a record number of competitors. The vast majority will be foreigners, with many coming from the US.

An entry fee of 1,264 yuan (HK$1,400) makes the race too expensive for most locals. When hotels and flights are added, the cost of running the marathon excludes all but the richest Chinese. Many local runners feel that's unfair.

'The organisers should try to make an effort to get more Chinese to run; after all, the race is in China,' says Guo. 'They should make the entry fee less for Chinese people.'

But the event has become something of a cash cow for the Tianjin municipality. Not only does the local tourism bureau get 200,000 yuan for closing the Huangyaguan section of the wall for a day - far more than it would get from the tourists who visit daily - but local hotels and restaurants benefit, too, from the annual influx of runners and their relatives and friends.

All of which means there is little incentive to attract domestic competitors. There isn't even a Chinese website for the race, with runners having to book through accredited travel agencies in their home countries. 'It's a strange one,' Gratton concedes. 'Being out on the wall is fantastic but, apart from our Chinese guide, we don't meet many Chinese people.'

For the past nine years, the race has been hosted by the Beijing Weiwen Hengshi Tourism Company. Its owner, Guo Feng, defends the high entrance fee, saying he gives discounts to regular Chinese runners and charges people from the Tianjin area only 200 yuan. 'We have 300-400 staff on duty for the marathon and we provide five foreign doctors as well. All of that costs a lot,' he says.

Potential environmental damage is another problem. 'I never throw away anything like water bottles when I'm running the marathon, no matter how hard that is,' says Huang Qing, a 35-year-old sales manager from Dalian. 'But I've seen a few foreigners doing it.'

Huang has run 33 marathons, including Boston's, and will be running the Great Wall for the fourth time this year. 'I enjoy seeing the villages and wild places. Normal marathons are all on the road and they almost all look the same,' he says.

Guo Ying says it's the local people who are the attraction. 'For the people in the nearby villages it's a big event and they all come out and cheer. I think they like to see a Chinese lady running,' she says.

More women are taking part in the race. Gratton says half the runners from Britain will be female. 'It used to be only about 10 per cent but now it's 50-50. They're getting older, too. The average age of our runners is 45. But we once took a 70-year-old. He was disappointed he didn't get a prize for being so old,' he says.

Given the extreme nature of the course, times are almost irrelevant. The record is 3 hours, 23 minutes, set last year by Spaniard Salvador Calvo, 44. Most people, though, will take an hour or two longer to finish than they would a street marathon. 'Any time below five hours will be good for me,' says Rueegg.

A manager at an insurance company, the Swiss moved to Hong Kong last year and has been training by running the trails on Lantau Island.

Shaw-Gray has a bet with her students on how she'll do. 'They're all trying to guess my time. I reckon somewhere between five and six hours, but maybe that's optimistic,' she says. 'They think it's fantastic that I'm doing it, but I'm not sure they understand the full magnitude of what's involved.'

However, Shaw-Gray's confident she'll complete the course. 'I'll finish it, even if I have to crawl across the finish line.'

She may not be the only one.