I inch along the jungle path, heat sapping my strength. The boils on my hand are throbbing; sweat pours into my eyes. And that’s when I see them. A group of men crouching in the undergrowth. Their leader is signalling our presence to the others. It’s an ambush.

As they rip from the shadows, shouting in Spanish, I desert immediately and at speed. I plunge into the greenery, spines gouging my hands and face. I hear my friends being captured. Water fills my mouth and nose, my boots and pack dragging me down like sandbags. I am out of my depth in more ways than one. There’s a chance there are anacondas in this river. The vegetation is poisonous to the touch, and if I don’t drown, the likelihood of being ravaged by disease-infested mosquitoes is high. What’s my plan? As the strength leaves my body, someone grabs me, shackling my hands with cable ties. A hood is placed over my head. I don’t protest as I’m dragged out by my thumbs; it’s a relief.

This cat-and-mouse game is all part of a much bigger game. I have been flown to the Amazon to undergo SAS training with ex-special forces operatives, who are also the stars of a reality gameshow. As a writer who works from home, I have perhaps the most sedentary job known to man (my pleasures usually involve snacks and stationery), so this is going to be a challenge.

My kidnappers, Ecuadorian locals, have been briefed. Presumably, insurance paperwork has been filled out. But I can’t help feeling that the piranhas, venom-spitting snakes, rabid vampire bats and one weird-looking black toad don’t know the plan. The water in my mouth is real; the danger feels that way, too.

Why am I drawn to SAS: Who Dares Wins? I loathe machismo, scorn exercise

Survival television has been upping the ante lately. From the start, the format emphasised toughness: remember Bear Grylls using a sheep for a sleeping bag, or Ray Mears eating pig nuts and sleeping rough in his canoe? Now there is a clutch of far more masochistic shows, to which I am addicted, despite my better judgment. Currently airing is a new series of Hunted, in which ordinary people make themselves fugitives, living off-grid to evade capture by a team of manhunters. There is Eden, a year-long experiment in which participants try to build a new society from scratch, isolated from their families and the modern world.

The most gruelling, though, is the one I’m about to experience – SAS: Who Dares Wins, about to start its second series. The show puts 25 civilians through an intense course that recreates the UK special forces selection programme. This is the most challenging military entrance exam in the world, for a unit that is the model for all elite forces. Open only to serving soldiers, it has a dropout rate of more than 90%. In the TV version, contestants undergo enhanced interrogation, involving sleep and food deprivation, as well as ceaseless physical punishment administered by men with neck tattoos. What I have experienced so far is just a taste, and it doesn’t taste very nice.

The fear begins weeks before, when I have nine injections, one of which contains live yellow fever virus. The nurse points to a map of the area of Ecuador I’ll be flying to: it’s the deepest red imaginable – red meaning risk. Aggressive daytime mosquitoes carry zika, chikungunya and dengue, while malaria is a high risk at night. “You won’t have time to build immunity to rabies, so don’t get rabies,” she advises, not altogether helpfully. “Because it’s fatal.”

‘Where did this cultural appetite for self-punishment come from?’ Photograph: Carlos Noriega/The Guardian

The immunisations are followed by a psych evaluation in which a mental-health professional advises me on the likely fallout of the trip. “You’ll be disoriented, possibly depressed. It will be harder for the people you live with,” he says, which is the closest thing to comfort he offers. I’m questioned about my mental health history, the stability of my relationships. I’m warned that being displaced and undergoing trauma may lead to mood swings lasting weeks or months. My interrogators might dig up sensitive information and use it to manipulate me. Then there’s a physical exam in which I strap on a mask and power an exercise bike until my lungs burn. The doctor pronounces me of “below average” fitness. Not so unfit I can cancel (the sick note I was looking for), but unhealthy enough that I leave convinced I am going to die.

With admittedly circular logic, I am putting myself through this to understand why people put themselves through it. I know people want to be on TV, but why not Catchphrase or Deal Or No Deal, where you might win some money? The appeal of Hunted and Eden I understand: they’re a chance to challenge the power of the surveillance state, to rewind the alienating complexity of globalisation. But why am I drawn to SAS: Who Dares Wins? I loathe machismo, scorn exercise, and would laugh if presented with an Andy McNab novel, so what is it about lunkheads doing burpees until they’re sick that I find compelling? Where did this cultural appetite for self-punishment come from?

“We lead very regulated lives, where almost everything is done for us,” explains Dr Rhonda Cohen, a sport psychologist at Middlesex University. “But knowing we can survive is a very important part of being human. We crave the feeling of being alive that comes with breaking routines and overcoming challenges. It’s extremely satisfying to use our bodies in new ways, and important developmentally. Kids get that feeling of reward all the time, because they’re always learning new things. It’s harder as adults.”

'Macho guys who come to prove a point about their hardness usually quit first, having proven nothing' Jason Fox

This urge to push our physical limits can be seen in the rise in popularity of what I call lunatic or non-fun runs. Since 2011, there has been a 20% increase in people doing ultra marathons and extreme obstacle courses. There are plenty to choose from: Thames Path 100, Spartan Race, the Spine Race (268 miles along the Pennines in winter). There are Ironman, double Ironman, Triple Deca Ironman triathlons. Tough Mudder, which involves only 12 miles of running, compensates by filling those miles with electric shocks, tear gas and rings of fire. As unimaginable feats become commonplace, elite competitors are seeking more extreme settings. The Marathon Des Sables takes place in 50-degree heat in the Sahara, while the 135-mile Badwater course runs from Death Valley, the lowest point in the contiguous United States, to Mount Whitney, the highest.

Meanwhile, spending on gym membership has risen by 44% in the UK in the last two years, and there has been a parallel increase in free, outdoor workouts such as park runs, wild swimming and cycling. We’ve changed the way we work out, too, moving from repetitive, machine-assisted exercises to more functional training. Where once we jogged sedately for hours, now we run, jump, squat and lift, movements the natural world once demanded of us.

Increasing numbers of people want to train like soldiers, the most functionally fit people on the planet. British Military Fitness runs 400 classes in 140 locations, and is adding more. “Athletes aren’t robust, they snap easily,” says Dr Sundeep Chohan, a medic who has worked extensively with the special forces and is dismissive of gym‑fit Instagram posers. “Military fitness is a combination of 20 factors – things like peak flow and blood pressure. These are supremely adaptable and robust people. Someone with a six‑pack won’t last long in the Antarctic, where you need body fat to survive.”

Chohan is here with me in the jungle. So is Jason Fox, aka Foxy, a former SAS demolitions expert and the most approachable of the directing staff on Who Dares Wins. (Directing staff, or DS, is how the instructors are addressed; they prefer this to Sadistic Inquisitors or Total Bastards.) Foxy puts another myth to bed, about the SAS and masculinity: “Macho guys who come to prove a point about their hardness usually quit first, having proven nothing,” he says. “A course like this strips you down to your base level, which a lot of people don’t want. They don’t want to know what their real character is. The people who get through need to explore themselves, or something that’s happened in their lives.” Foxy joined the SAS because he disliked bullies, craved autonomy and was “fed up with the pageantry and bollocks of the regular military”.

Matthew ‘Ollie’ Ollerton gives Rhik a few pointers. Photograph: Carlos Noriega/The Guardian

He is a gentle individual, with a lisp. It’s hard to remember, as we bounce down a dirt track in a truck, that he is a trained killer with exceptional military capabilities. Of the 350 men with whom he trained for SAS selection, only 14 made the cut. The other instructors are equally surprising. There’s Matthew “Ollie” Ollerton, who advises us on resisting interrogation. (Humanise yourself; practise the controlled release of information; never answer questions with a simple yes or no, because this can be manipulated into a recorded confession.) Ollie is referred to by another instructor as the “short-haired hippy”, and at one point I spot a whiteboard on which he has written: “When ego is lost, universality exists.” Jason and Ollie run Break-Point, a company that teaches special forces techniques on corporate awaydays, which I would happily pay to watch.

Then there’s Mark “Billy” Billingham, grizzled, blue-eyed and scary, the sort of man I imagine Gordon Ramsay fantasises about being. Billy is a spotter: crossing a line of leafcutter ants, he explains that humans are the only animals clumsy enough to break it. “Everything else will step over. If you see a broken column, there’s an enemy less than 18 to 20 minutes ahead of you – which is how long it takes the ants to reform.” Billy teaches us that immersion in the jungle eventually makes your skin reek of urine, due to the production of ammonia, “but you start to blend into your surroundings. I can smell soap on a person half a kilometre away.” He tells us his training turned around an unpromising childhood on a West Midlands council estate, where he was stabbed at the age of 11. “Suddenly I’m learning Serbo-Croat, performing throat surgery in the field, advising prime ministers.”

Who Dares Wins features no weekly eviction; in the first series, there was no overall winner. Contestants simply try to endure: evading capture, rescuing casualties, being subjected to surprise interrogations and mind games, supporting each other through pain. Like Billy, many are aiming to turn their lives around, or overcome traumatic experiences. One contestant in the new series signed up after his son was killed in Afghanistan, having recently begun his own SAS training. Another, a young British Muslim who has suffered racist attacks, wants to prove himself as a role model. Reformed gamblers and ex-gang members rub shoulders with posh boys who want to be shorn of their privilege. It’s much more than vicarious thrills; whole life stories are being played out.

‘I’d like this to stop. My thoughts run away with me. Maybe I should change my job. Or move house.’ Photograph: Carlos Noriega/The Guardian

Even so, I have to admit something makes me uneasy about survival television. It’s a strange thing to do, passively watch other people absorb punishment. How much further can shows such as this go? Are we headed for The Hunger Games? The SAS selection process was last year the subject of a judicial inquiry after three soldiers died on the same training march in 2013. The deaths were attributed to neglect, but another death followed in July this year, on another SAS march in the Brecon Beacons. Tests that explore the limits of human endurance may be necessary for those who want to join an elite fighting unit; it’s less obvious that civilians need go anywhere near them. The ultra-fitness craze is supremely at odds with the comforts of the first world: what are we compensating for?

***

I’m marching through the jungle with a bag over my head, blackout goggles strapped over the top. It is 35C, the humidity 95%. I stumble over branches and snag myself on thorns, to the irritation of my captors. Every step feels irrationally as if I’m being walked over the edge of a cliff; and then they make me jog faster, totally blind. I’d like this to stop, I think. Semi-delirious, I no longer feel like myself and my thoughts run away with me. Maybe I should change my job. Or move house. Maybe I will run into a tree and be granted unconsciousness. The only way for a contestant voluntarily to withdraw is to remove their numbered armband and hold it in the air. Due to an acute lack of bicep, my armband has long since slipped off my arm and been lost. The feeling is akin to being chained in a dungeon with no safe word. (Probably.)

Soon, that’s almost exactly where I find myself, when I’m dragged to a cell and made to hold various stress positions. These are unnatural poses in which one’s bodyweight is balanced on single muscles or joints – holding a deep squat for long periods, for instance. Even keeping my arms above my head proves agonising after 10 minutes. Threats are whispered in Spanish while my body convulses with pain. I’ve reached my limit. Today I have hung from ropes surrounded by hornets, holding my entire bodyweight until my arms crumpled. I’ve scrambled up banks through thick vegetation. There are vines to support me when both legs fail, but they bring the palms up in stinging boils, prompting a tricky cost/benefit analysis. I have fallen down steep climbs, slid face first through enormous webs, and now I can’t take any more.

I give nothing away. The two men seem impressed by my resilience. The reality is that my brain has checked out

At this point, I am interrogated by Billy and another soldier, in scary mode, screaming questions in my face. I can remember neither the mission I am supposed to be on (something about reconnaissance on a drug plant), nor the cover story we invented. I know I have some names and coordinates on a square of notepaper tucked under my testicles, which I don’t want anyone to go looking for. Even when the hood is lifted and I find I am kneeling at the edge of a shallow grave, I give nothing away. The two men seem impressed by my resilience. The reality is that my brain has checked out. Yet I’ve been here only one day; the real competitors endure this quasi-torture for three sleepless days, spending 10 days in all pushed to their limits; when Foxy, Ollie and Billy went through real SAS selection, they did this for six months.

Reality TV can never replicate these former soldiers’ experiences. In the first season of Who Dares Wins, Foxy revealed he was discharged from the SAS after suffering from PTSD. How does he feel about military training being mined for entertainment? “It’s education and exploration first,” he says, thoughtfully. “It’s about showing people that our humanity gets us through tough moments, not about being tough. We’re not invincible superheroes, and people can relate to that. It drives them to do better for themselves.”

‘Afterwards I relive the few moments when I didn’t let myself down. Scaling a tower of logs and throwing myself into brown water’ Photograph: Carlos Noriega/The Guardian

It’s hard to make the connection between physical achievement and emotional satisfaction if you haven’t experienced it; but, once you have, it’s hard to break. Back in the UK, after my arms and legs and face have recovered, I find myself mentally reliving the few moments when I didn’t let myself down. Scaling a tower of logs and throwing myself into deep brown water from the top. Accurately recalling the location of our camp when questioned, out of breath and disoriented. Spying the kidnappers early, gaining a time advantage and having the confidence to brave the bush. Even if it was just for a minute, I freed myself. I am left with an entirely unexpected feeling: pride.

A few months later, I am turning into one of the people I hate. Someone who does exercise, and even worse, talks about it. I’ve started running, and swimming. I’ve started running around my local park. On holiday in France I circumnavigated a lake, clearing 8km without realising it, then 10km. I’m wondering about a half marathon. It helps clear negative thoughts, gives me a sense of self-reliance, a scale of achievements I can be in control of reaching.

I finally understand the value of exercise, and all it took was a simulated kidnapping and some piranhas. Given the choice, though, I’d still choose Deal Or No Deal and a large cash prize. No contest

• SAS: Who Dares Wins begins on 17 October, Channel 4, 9pm