Men and horses jockey for position over 22 miles

Every year, a small Welsh town holds one of the world's stranger marathons that pits humans against equine competition. It's not as far-fetched as it sounds, thanks to rather a lot of steep hills.

An elderly man's voice crackles over a loudspeaker in the town square. "Today is a very good day to beat a horse," he says. Before anyone calls the RSPCA, this is not a call for animal flogging. We are at Llanwrtyd Wells, in the Welsh valleys, which each year hosts a marathon pitting more than 300 men and women against 50 horses over a gruelling 22-mile course.

Gordon Green, the man on the loudspeaker, should know a thing or two about horse-beating conditions. The 75-year-old dreamt up the Man vs Horse Marathon in 1980 during an argument in his pub – he was the landlord – the Neuadd Arms. One punter insisted that, over a long and hilly enough track, a man could beat a horse. And so Green decided to put it to the test. Every year since, this small Welsh town has hosted one of the nation's strangest sporting events.

And the reason that this is a particularly good day to beat a horse? The weather. In 30 years, the horses have been beaten by men only twice – and on both occasions it was swelteringly hot. As the thermometer climbs into the twenties, excitement builds that man might just win again.

The run's route – which starts with a slope so steep that its green face overshadows the rooftops of the town centre – is so relentlessly hilly that most sane men and beasts would avoid having anything to do with it. Fortunately, sane people are in short supply. A representative from the Monster Raving Loony Party is doing the rounds in honour of Screaming Lord Sutch, who used to support the event, and most of the runners are blessed with healthy doses of eccentricity.

One Brighton couple has decided to confuse matters and run the race dressed as a horse and jockey. Ian Parsons, 42, sports a very hot-looking, furry horse's head, while his girlfriend, Evie Cicha, 35, stands on the starting line in full riding regalia, including a riding crop which she occasionally aims in her "horse's" direction. "This way I stand half a chance of beating the horse," giggles Cicha.

Others are taking the affair altogether more seriously. John Terry, 33, is here for the fourth time. Now he wants to beat his personal best. "I got within four minutes of beating the horse in 2008 and came 12th overall. That was two hours and 46 minutes: I'd love to beat that."

With a Cape Canaveral-style countdown of the last five seconds, the runners are off; pelting with impressive enthusiasm out of the town square and up the first hill. Just 10 minutes later, a clattering of hooves signals that the horses – ridden by amateur enthusiasts – are in the square and ready to begin the chase. The starts are staggered by 15 minutes to avoid too much trampling.

It took 25 years for someone to beat the horses. Huw Lobb, a British marathon runner, and took home the then cumulative prize of £25,000. Since then neither he nor the previous sponsors, William Hill, have been involved in the race. "I don't think they fancied putting in the money once they realised a man could win," laughs Green.

A third of the way around, at Abergwesyn, it looks as if the weather really has given humans the advantage. Staggering down the steep slope, looking about as competent as fawns taking their first steps, the horses demonstrate just how the human runners can make up time downhill.

But sadly, and perhaps thanks to enormous buckets of cold water being thrown over the mounts as they pass, the horses gallop down roads overtaking a stream of exhausted-looking and sunburnt runners.

The only man not burdened down by the heat is Haggi Chepkwony, a lean 40-year-old from Kenya who lives in Bristol. He runs a good five minutes ahead of the others with not so much as a bead of sweat on his face.

Runners and horses begin to jostle for position. An exasperated man on horseback shouts: "HORSE TO YOUR LEFT!" and a shocked runner stumbles out the way – so tired he looks as if he'd entirely forgotten his four-legged competitors.

At the finish line, a grassy enclosure just outside the town, excitement builds: Chepkwony may be on track to do what only two men have done before him in 30 years.

Striding over the finish line after two hours and 17 minutes, he smiles, then stands still and begins the agonising wait. To win he must be at the finish line for at least 14 minutes and 25 seconds before any of the beasts arrive, he explains. He has hardly finished speaking when there is a distant thunder of hooves. He starts: "Is that the horse?" and his face falls.

A minute later and his fears are confirmed as Llinos Jones, 24, and her mount, Sly Dai, appear over the brow of the hill. They have done the race 10 minutes faster.

Chepkwony, who still hasn't even had a sip of water, stares, crestfallen. "I guess I'll just have to train more next year," he says. Jones, meanwhile, is jubilant. She leaps from horse with a flourish, shouting to her gathered friends: "Where's the fucking Strongbow?"