The growing attraction of pushing through the pain barrier
"I loved it. I had a grin on my face the whole way round," is how Gwen Hamilton, a 43-year-old mother of two, says she felt after doing what the organiser bills as the "toughest event on earth" earlier this year.
The Edinburgh race she did involves running 12km (7.5 miles), which sounds simple enough, until you consider the 30 obstacles en route which include jumping over fire, crawling through piles of deep mud, plunging into freezing water and clambering over 10ft walls.
For weeks afterwards Ms Hamilton says she was still getting mud out of her scalp, and finger and toenails. It's a far cry from her desk-based marketing job for the Scottish government.
This, and the fact it was a challenge, was the appeal. Now, she admits, she's "totally addicted", and plans to do it again next year. "I know I can do it. Now I want to see if I can do it faster," she says.
Ms Hamilton, who'd previously done triathlons and played hockey but saw the race as a "step up", is part of a growing minority doing more extreme events.
The rise in people willing to put themselves through these kinds of challenges means what was once the domain of small, amateur operations, is becoming big business with numerous companies setting up to take advantage of the burgeoning interest.
Obstacle race firm Total Warrior, which organised the race Ms Hamilton did, is a case in point. The first competition the firm ran in 2011 in the Lake District attracted 2,000 entrants. This year it had 7,500 participants, and the firm now runs three separate events in the UK, and has an annual turnover of £800,000.
But while the gains appear lucrative, the firm's co-founder Andrew Murray admits that if he'd known what he knows now he would never have started the business.
As well as the big costs of land rental and insurance, putting on the events involves hiring a huge number of contractors; including a structural engineer to design the obstacles, builders to construct them, a health and safety manager to ensure the finished structure is safe, and a doctor who looks for any injury correlations.
The costs are "just incredibly high", says Mr Murray, who says if they'd had less than 2,000 entrants to their first event they'd have been in financial trouble. To ensure the firm had sufficient cash to keep operating, they opened entries to their next event immediately.
"We needed so many people to make money. Fortunately everyone pays you before event day, but all your suppliers bill you after event day. From a funding point of view we were very lucky."
The other key element in organising races like this is of course safety. The firm's health and safety manual is 200 pages, and Mr Murray, whose background is in tennis coaching, says their first meeting with an insurance company was a "rude awakening".
So far, they've had four or five broken legs, a dislocated shoulder and a few twisted ankles, which in the context of the 40,000 people who've participated so far is "a pretty good safety record", assures Mr Murray.
He says much of what the firm does is create the "perception" of risk, whilst in reality ensuring there is almost none.
Given the difficulties, he doesn't expect the number of operators to continue to expand, predicting instead a handful of firms running big events, and small local races continuing at an amateur level. "We know how much money you need to make it work," he says.
Simon Murie, founder of SwimTrek which runs tours where people swim from place to place with the tagline "Ferries are for wimps, let's swim!" agrees, saying running this kind of firm is still primarily "a lifestyle business" with limited financial rewards.
Last year the firm made around £140,000 profit on sales of £1.4m, but for the first decade profit was "very modest", he says.
For Mr Murie the biggest hurdle has been getting suppliers on board, from boat pilots to accommodation providers. On the firm's first trip to The Cyclades - a group of Greek islands - when the tour group didn't arrive on the once-a-day ferry, because they were of course swimming there, the hotelier let out all their rooms.
"Educating suppliers is harder than getting clients," he says.
Nonetheless, he says it's worth it. "I'm doing something I love and it's always nice and fulfilling to turn your passion into your work."
British Military Fitness (BMF), which runs army style fitness classes in parks across the UK, as well as a series of obstacle races, was also a pioneer when it began in April 1999.
Classes continue in heavy rain, or even snow, with participants required to do things such as carry logs, or run repeatedly up muddy slopes while instructors "encourage loudly" as co-founder Harry Sowerby puts it.
Back in 1999 fitness classes were typically gym based and getting people to pay from £27 a month to do tough old school exercises, such as press ups and squats, in parks that they could otherwise use for free was a long shot.
Just three people turned up to BMF's first class in London's Hyde Park, but it now has around 15,000 members.
Yet the high costs of licences to operate in the parks - around £1,600 for a London park for example - plus equipment and instructor fees meant the firm almost went bankrupt at one point, with over £200,000 in debt.
Now the firm's biggest challenge, alongside the increasing number of competitors, is to keep the spirit and camaraderie of its early days.
"With size came corporateness. We maybe lost our way a bit," says Mr Sowerby. "BMF is about having a laugh and getting fit. I think we can do both."
Managing to expand, while keeping the sense of adventure and uniqueness that attracted people to it in the first place, is now the challenge for the industry as a whole.
Ms Hamilton, however, says she won't be easily deterred. "I used to look at people doing things like this and think I could never do it. Now I've realised I can and it's a lovely feeling."