There is a woman ahead of me, climbing on all fours. Even in my salt-caked, exhausted, sweaty state, I don’t think that’s a great idea. If I bend down, I fear I’ll never stand up again. Or, even worse, I might sway over, and tumble off the side of the Wall,  down a hunded-foot drop. I grab my thighs in my hands and force them into action, taking the steps in sets of 10. Counting helps, sort of. But the number of giant stone steps rising vertically ahead never seems to dwindle. I take the steps in sets of 30. After each set, I pause to gasp, and curse the Chinese emperor who built the Great Wall. Then I curse the Danish travel agency which devised a marathon to be run atop it.

Way down below the Wall I can hear distant cheers from the finish line. This spurs me on. I am at 23 miles and have been running for just over five hours. Actually, forget running. Desperately scrambling is more like it.

The Greeks, who were the ones to first formulate the agony that is running 26.2 miles, said that humans were hard-wired for challenge. And that includes challenges with oneself. Hence the modern phenomenon which is Marathon Tourism. For once you have run a local marathon or two, on proper roads with friendly support and foil blankets, you start itching for a bit of jeopardy.  And jet lag. Why not do a marathon in Marrakech? Or Rio, or Tibet?  Trotting around the world in bursts of 26.2 miles, a sweatier version of Monroe-bagging, is big business these days.

On the bus from Beijing to Huangyaguan, Tianjin Province, the talk is all about who has had the most exotic marathon experience. Coen Verbec from Holland has run LA, Berlin, Lisbon, Rotterdam and Ankor Wat. Betsey and Dixie, both in their sixties, both from Charleston,  have run marathons on Mount Kilamanjaro and Athens. “Athens, Greece,” says Dixie helpfully, in case I get muddled up. “It’s a way of making you go there,” explains Colleen Surridge from New Zealand, who says she’s running Petra next year.

I meet people who have run marathons in deserts, on ice fields, up the Inca Trail and down the Sunset Strip. Non-runners might not agree, but trotting through a landscape, or city centre, is the perfect way to sight-see.You are on a human level and moving at a human speed, in the great outdoors. Usually. Zdenek Plachy, an IT specialist from Prague,  has run a marathon in the unlikely venue of an underground car park somewhere in the Czech Republic. “You go sixty nine times round. It’s very popular. It takes away the problem of bad weather and wind, you see,” he offers, somewhat unecessarily.

The Great Wall of China is up there in the Top 10 of great marathon runs. The crazy notion of running a marathon on the Great Wall of China was dreamed up by Danish travel agent Soren Rasmussen about 13 years ago, when he had an Eureka moment in his kitchen. Marathons were big, popular events; and the notion of Adventure Holidays was just coming into focus. Why not combine the two? Plus, everyone mythologises about hitting the ‘wall’ in a marathon. What a lark if you could hit the wall  - on the actual Wall! 200 runners signed up for the first race. The figure is now over ten times that, and Rasmussen has devised five or six other ‘Adventure Marathons’ (including one in Northern Australia which kicks off this year during a Solar Eclipse and thus claims to have an ‘Intergalactic Start Gun’). But even though the Wall is his toughest run, it’s the most popular. Thinking it might be nice to run a lonely, remote marathon, and with May a comfortingly long way off, I sign up.

“It’s just a long run with two big climbs at either end,” says Julie Sparrow, multiple marathonista and lead physiotherapist to the British Olympic gymnastics squad. She ran the GWC Marathon about eight years ago. How did she get on? “I blew up at 20 miles. So I sat on the Wall for 40 minutes, and ate three bags of Jelly Babies. Then I carried on.” A lot of runners must have gone past, I say. “Actually, nobody did,” says Julie, calmly. How long did she take to finish? “Oh, about seven and a half hours.”

I start to get very worried. This particular stretch of the Wall means conquering 5,164 steps. You must climb for 3 miles to get onto the Wall, charge up and down 2,500 steps, experience a 16-mile jaunt around Chinese villages (and some horror climbs), then do 2,500 steps on the Wall again, going  the other way. It’s nasty. These steps are both tall and tiny, designed to fox medieval invaders, but just as tough on tourists in Nikes. Still, the race is advertised as achievable for the ordinary runner, so why not?

“Do a LOT of stair running,” advises Julie. I visit local blocks of flats and run up and down staircases. I rashly boast about this on Twitter, only to be comprehensively bested by Idris Mohammed from Bahrain, who tweets that his Great Wall training involves running up and down multistorey car parks in ‘a weighted vest’.

My neighbour Ben,  whom I have inveigled to come on this madness with me, does hourly sessions on a Stair Master. My other neighbour Christian, who in a spirit of comradeship is coming too, does the London Marathon as a ‘warm up’.  We are taking it seriously. We are afraid. The Wall is a 2000-year old monster and has had many victims, (although nobody has actually expired on the Marathon).

I discover, during my 10-hour flight on British Airways to Beijing that the entertainment is suitably sporty. I watch Chariots of Fire, and then a documentary called The Spirit of the Marathon.  Hugh Hudson’s classic is far better, since it involves men in elegant outfits in races which are over in a matter of minutes. In contrast, the Marathon film features weeping women in Lycra,  whose agony goes on for over four hours. Four hours is nothing to the GWC Marathon. Four hours would be remarkable. Four hours is a doddle. This baby could  go on for almost a day.  “There’s an eight hour cut-off,” says Ben. “Look, we could probably walk it if we had to.” Funnily enough, this does not reassure me.

At Beijing Airport we meet David and Steve Jolliffe, 53-year old twins from Watford. They don’t look particularly fit. Why are you doing the Great Wall? I ask them. “It seemed more interesting than just having a holiday,” says David drolly.

This is another facet of marathon-bagging. Running a marathon abroad is a bit like volunteering to help orphans in Africa; both equate to a Virtuous Holiday. Quite why marathon running – with its arduous training and selfish requirements (obsessive mile counting, teetotalism, early doors) – has a moral equivalent to helping orphans, I don’t know, but it seems to.

After a day or two in Beijing, where we eat heroic quantities of noodles and one of the British runners – to his horror- is accosted in his room by a local masseuse in bra and knickers,  asking if he likes her “silicon implants”, we travel to Huangyaguan and the Wall. On the start line, there are 2300 of us; 700 nutters doing the full marathon, the rest doing shorter distances.

Eight or nine miles in, I am on my own, trotting down a rural road somewhere near the Great Wall, Tianjin Province, China. All I can hear is the rustling of my paper number pinned onto my shirt, my footsteps, and my breathing. It is 80 degrees. From the middle of a rice field, a crouching farmer in a straw hat stands up and watches me go past. Two small children run out into the road, waving. Far away I can see a petrol station. Giant mountains rear up around me,  carpeted in forest.  For anyone who has experienced the razzmatazz and sheer volume of bodies, noise and hoopla in London’s big race,  there is surely no stranger marathon experience.

Fourteen miles on,  and its carnage. The mighty Wall and its thousands of fearsomely steep steps must be conquered. Again. People are dropping as they run, and being carried down on stretchers to a field hospital, where they are put on drips. Christian’s knee gives way (he later tells me). Ben vomits on the Wall (he later tells me), and temporarily passes out in a medieval Watchtower. I sprint off the Wall and hit the Wall. My legs simply won’t work. “Come on Rosie!” yells Zdenek, hobbling past. 119 runners never make it.  But 581 of us do, including the Jolliffe twins, Idris Mohammed and both my neighbours. I cross the Finish line after 5 hours and 41 minutes. Never again, I vow, grasping my medal and smiling through sweaty tears.  Although, the next day, I start thinking about the idea of Petra….

www.adventure-marathon.com

FROM COUCH TO GREAT WALL- Rosie’s Training Regime in 5 months