While I’m not exactly hooked, I find myself being drawn in, intrigued to find out more about this world of ultra running. A few weeks after Oman, I get to meet Elisabet again. I’ve been commissioned to interview her for an article for the Guardian, so she comes to meet me at the newspaper’s offices in central London, where we sit on leather sofas, drinking espresso as people queue to buy their lunchtime sandwiches. I ask her how she first started ultra running.

She tells me she used to be a keen marathon runner, fitting her training in around a well-paid job in London’s financial district. ‘I kept improving,’ she says. ‘But there came a point when I thought, what shall I do now? I could run a marathon faster – which is, of course, difficult, and a good challenge – or I could go further. And I just thought it would be interesting to explore the going further.’

Her decision to delve into ultra running was accelerated by some unexpected life events. Within a short span of time her father died, her mother was diagnosed with Alzheimer’s, and her husband got cancer. ‘All of those things,’ she says, ‘make you realise that life is very short, and you just have to do it, you can’t sit and wait.’

So she quit her job in the City and set off in search of adventure. To fund her way she set up her running shop, but since winning the MdS and now the Oman race, people have started writing articles about her, and sponsorship deals have begun to come her way. This makes it easier for her to travel to races and take on more challenges. What started off as a risky move seems to be paying off.

Listening to her talk, I too feel a stirring. It reminds me of the feeling I had when I first decided to run a marathon. The idea had been hovering on the horizon for years, watching me as I tackled shorter races, wondering what was taking me so long. Suddenly, it was time. Life was moving on. So I ran a marathon.

Ever since then, on the horizon, I keep catching glimpses of a trail up a mountain. A long, winding trail. I’m forty-two years old. I’ve had a few decent attempts at the marathon. Maybe it is time to explore the going further. To explore just what it is that people find out there on that trail that compels them to run these improbable distances.

It’s so intriguing that I’m soon on the phone to my editor. ‘I think I’ve found my next subject,’ I say. I’ve written books about travelling to Kenya and Japan to explore two unique running cultures. This time I’m drawn to investigate a cross-cultural, global phenomenon, which I’m only just beginning to realise is so huge. What is this world of ultra running? Who are the people taking part? What is it all about? The best way to find out, I decide, is to sign up for another race.

Over the last decade, ultra running has grown at a staggering rate, becoming one of the fastest-growing sports in the world.

The website runultra.co.uk lists most of the world’s biggest ultra marathons. Its founder, Steve Diederich, tells me that when he set it up twelve years ago he found 160 races listed globally. He now has over 1,800 races on the site – an increase of over 1,000 per cent. The German ultra running website DUV, meanwhile, lists the results of many smaller races, with its forensic database going all the way back to the first 89km London to Brighton race in 1837. Over the last ten years the site plots a similar 1,000 per cent increase in the number of ultra races around the world.

Andy Nuttall, the editor of ULTRA magazine, drilled down deeper into the DUV statistics and found that in the UK the rise of the sport has been even steeper: in 2000, only 595 people finished an ultra marathon in the UK. By 2017, the number of finishers had grown to 18,611.

Everywhere I look, it’s a similar story. Ultra Running magazine in the US collects figures for North America, which show the number of races and finishers increasing every year since 1981. In Asia, too, the number of ultra races has exploded. Nic Tinworth, a race director in Hong Kong, tells me that ten years ago there were six ultra races in the territory, but that there are now over sixty. ‘In previous years,’ he says, ‘you could just turn up on the day in Hong Kong and enter, but now the most popular races sell out in minutes.’

Many of the world’s most over-subscribed races, such as the Ultra-Trail du Mont-Blanc in France and the Western States 100 in the US, have had to implement lottery systems to cope with the numbers wanting to take part. Diederich manages the UK entries for the Marathon des Sables. Despite the steep £4,250 entry fee, he says the race sells out each year in a couple of minutes.

What are all these runners striving for? I experienced something transformative in Oman that stayed with me long after the race. But I sense there is more to discover. I fell apart over the last two stages and almost gave up. Imagine if I could stay strong, even in the face of such a challenge.

I remember being struck by a photograph I saw of the Spanish ultra athlete Azara García, who has a tattoo on her leg that reads (in Spanish):

Is this the appeal of ultra running? To push ourselves to a place where we stand face to face with the Devil, the depths of the struggle, but then to rise up and overcome it? Could I stare into the storm – whatever it may be, whatever is thrown at me – and overwhelm it with the force of my will? It is an enticing thought. And a far cry from the FT journalist complaining because his bus to the hotel is late.

I have to admit, this is all very appealing to my ego. I find myself watching a documentary about human evolution and the role running played in it, and a Professor of Anthropology at Hunter College in New York says: ‘We even have on record humans who can run 100 miles in one go.’ He says it as if it is scarcely credible, like these must be some kind of super-humans. And I spy my ego glancing over at me with that cocksure look that says: you could do that.

As the US comedian and ultra runner Michelle Wolf puts it in an interview with Runner’s World magazine: ‘It does kind of make you feel like a badass.’

Talking to other ultra runners, however, I get the feeling that it isn’t only the satisfaction and validation of overcoming the challenge and making it to the finish that appeals to them, but also the feeling of oblivion they get from striding into the midst of the storm in the first place, from teetering so close to the edge. Digging in the pain cave, as seasoned ultra runners describe it with relish.

When I start scouting around for races to enter, I find myself looking at the race profiles, and each time I feel a ripple of fear in my stomach. It seems that every ultra race has to produce a slick, short film with dramatic, sweeping shots and lots of high drama. And always, at some point, it shows someone looking broken, close to tears. The runners look more like survivors of some near-apocalyptic disaster than sportsmen and women. It is telling that these are the images they choose to advertise the race. People want to experience this despair, they want to get this close to their own self-destruction.

Many ultra runners tell me they were inspired to take up the sport after reading Dean Karnazes’s first book, Ultra Marathon Man. In it he describes in minute detail the process of being broken by a 100-mile race, with one thing after another failing in his body and mind until it seems nothing is left and he is literally crawling along the road on his hands and knees. I read it and I shudder. I don’t want to hurt that badly. But other runners say they read that and thought: ‘That’s what I want.’

So, a little scared, but with my ego convinced that I’m tough enough, I begin to search for a race that will give me the full experience of ultra running, that will deliver me into the heart of this growing sport, that will reveal the secrets and allow me to fully understand what is going on.

It is a big, unwieldy thing to get my head around, a sport morphing in many directions at once. With no central overseeing body or organisation, the races, interest groups and self-appointed guardians of ultra running jostle and fight for control and for a cut of the increasing amounts of money sloshing around. It’s a Wild West of a sport, still untamed, with many of the original prospectors fiercely protective of it, pushing back against the encroachment of ‘brands’ and ‘outsiders’ – people, they feel, who don’t understand the ethos of the sport. For many, part of the appeal of ultra running is its low-key, ‘Into the Wild’ minimalism, a chance to lose yourself in the wilderness, to cross the toughest, most extreme environments on Earth with little more than a flask of water and a rain jacket.

For some seasoned ultra runners, the influx of newcomers is already too much, and they are turning their backs on the big races in search of more isolated challenges. One outlet for these people, who hate mass starts and goody bags even more than a long night of hypothermia clinging to the side of a frozen rock face, is another growing phenomenon called FKTs (Fastest...