In the first of our new feature 'What it's like to...' Editor-in-Chief Susannah Taylor discovers what it's like to train insane at Barry's Bootcamp in London

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So I consider myself to be reasonably fit - I train roughly four times a week these days (I know, I can't quite believe that either), and like to think that I could do any class that I was thrown into. So it was with stupidity and cockiness that I threw myself into Barry's Bootcamp last week, thinking I would nail it. Tagged as 'The Best Workout in the World', Barry's (as it's known to its regulars), is said to burn 1000 calories an hour. I'm not sure who Barry is but it has come over from the US and has an insane cult following not only by mere mortals but also A-List celebs. Victoria and David Beckham are said to be regulars in London and Guy Ritchie was hidden in my class the day I went. So what's all the fuss about?

A session at Barry's is hardcore. If you suffer from migraines, prefer classical music to banging house, or gentle leg raises over high intensity training  I'd steer clear. Barry's is done in the dark with about 40 other people wearing their skimps, red lights overhead, thumping music and a trainer shouting into a headset.  It's well known for being the ultimate hardcore, sweat-inducing, lung-busting workout ever invented. If you are seriously hardcore you can even get involved in 'Hellweek' which is 5-7 days of straight classes. Gulp.

A sucker for punishment, I dragged my uber fit friend Rachel along with me since I was way too scared to 'Barry' it alone.  As we walked through the doors at 9.30 last Wednesday morning we knew this wasn't for the faint hearted - hit by a sea of just-Barry'd bodies, two men with bodies sculpted like Michelangelo sculptures stood in our way, rivers of sweat running off their skin wearing nothing but a pair of trainers and extremely tight Lycra pants. I couldn't help but feel a bit anxious.

"You're floor 3," said the ridiculously happy, bicep rippling guy behind the desk. Immediately Rachel and I got in the lift to go to floor 3. This was Barry mistake number 1 - what he'd meant was my number on the floor in the workout room (there was no actual third floor). Feeling a bit sheepish and very confused we found our places on the floor in the workout room just as the lights went down - at Barry's, half the room do floor work (weights, press-ups, crunches etc) and half hit the treadmills before you swap over twice. Sounds easy? It damn well isn't. I realised quickly that it's very necessary to tune your ears into the personal trainer (Sandy, below) as he shouts out orders over the music - otherwise he genuinely gets cross with you and you also get lost in the dark. "Come on you two, get moving and start listening," he barked. Mistake number 2, don't chat or joke with your mates at Barry's.

I found the floor work OK (I think I could have gone up in weights but wasn't going to tell Sergeant Sandy that), but it was on the treadmills that the fear set in. These aren't your usual gym treadmills - these are extra wide and have a tank-like tread to them for added grip. You only have to look at them to know you're running straight into hell.  "Start at level 5," said Sandy, which seemed doable. A minute later he shouted "Go to 7, 8 or 9" and I started to panic I'd fly off the end with Bridget Jones panache. Next, "Go to 10,11,12 and sprint as fast as you've ever sprinted before, break your RECCCOOOORDDDDD!" he hollered, before adding "Add an incline of 6". Whoever Barry was, I hated him at this point. I'm used to pushing myself, but I've never sprinted at a pace like this on a treadmill before  - it felt a bit like being on a rollercoaster you can't get off - and at one point I was so scared, so at my lung and leg limit that I jumped onto the side, white knuckled and gripping onto Rachel's machine for fear of my life.

The music got louder, heavier, the sweat started to fly, and Sandy got more intense as we did the whole sequence again (only harder), and as the session drew to a close I found myself sprinting harder and faster than I'd ever done in my entire life - the sort of sprint you'd do if you were being chased by a hurd of male elephants. At this point I realised too why it's a celebrity favourite - you can sweat buckets and look ugly as hell with your hair stuck to your beetroot face yet remain totally anonymous in the darkness.

There's also something about working out in the black room that makes you work much harder, knowing no one is watching or judging you (a major problem with the voyeuristic vibe in gyms). As we came out, shell-shocked, sweating and blinking into the daylight, Rachel and I sipped our Barry's smoothies (you order one of your choice pre-workout and collect it at The Fuel Bar the end), and we looked at each other, totally stunned into complete silence. Our brains and mouths started to slowly refunction when we met Sandy, who we discovered was also the owner (he was actually very normal and unshouty without his headset), and I explained that I just wasn't used to running like a total maniac. "No one is," he said, "It takes you way out of your comfort zone, but that's good. People get addicted to that."

Did we love it or hate it? I'm still undecided but something makes me want to go back to see if I can do it better. Sandy was right, it's horrendously, horribly hard, but there's something about it that makes you want to return to torture yourself all over again. Mistake number 3 - endure Barry's once and you'll be back.

Follow Susannah on social media  @STaylorGTG