The definitive guide to why gyms are terrifying if you are not 22 years old.

A lot can change in three years. Children can grow, Justin Bieber can become cool, Australia can cycle through 25 Prime Ministers…

Until yesterday morning at 9am, I had not set foot in a gym for three years.

If you are clutching your medicine ball in horror, I feel you, but don’t call the doctor. I have done some exercise in the last three years. Just not in the gym. And, you know, not much.

But things have been getting out of hand. If I sit cross-legged on the floor to play with my kids, when I stand up I hobble like I am 92. I am not 92. My regular “runs” have become less like runs, and less regular. The stairs at work leave me breathless. My lower back aches when I wake up.

So. You know. IT’S TIME.

And so. I went to the gym near my house and signed an iPad with my finger and gave them my credit card details and then I went and found my old smelly workout clothes at the bottom of a drawer and I walked into what used to be my very favourite class – SPIN.

Not Soul Cycle, or Life Cycle, or even RPM. Just Spin. A load of stationary bikes in a darkened room with very loud music and a trainer who yells at you. Good times. I like to think of it as clubbing for old people.

Twenty-four hours later and I have barely recovered. And not (just) from the exertion. (Post continues after gallery.)

Let’s just say, things have changed at the gym. Things like:

People dress up.

If you’re under 30, you might not believe this. But people used to wear baggy T-shirts and leggings from Best And Less when they exercised. Free promotional clothes, their flatmate’s old Singha singlet, that was the stuff you sweated in. A pair of LuLuLemon tights cost more than the dress I wear to weddings. My sweaty self is going nowhere near those things.

And they take pictures.

It’s hard for me to express how good-looking the woman on the spin bike in front of me was. She was so gorgeous, in nothing but her sports bra and pale grey LuLus (pale grey, with not a sweat patch on them, is she human?), with her tasteful star side-boob tats and her immaculate French braid swinging across the hill climbs. Her bum was so perfect and her legs were so long and she was RIGHT IN FRONT ME. I could barely look away, and was tempted to take a picture, just to prove her existence. Luckily, she was taking lots of those herself. So, afterwards, I tried it:

This is MY post-gym selfie. HOT HOT HOT. Actually, I was fricking boiling. 

There were CHILDREN in there.

In spin class. A boy, who looked about 11. Turns out, he's a member, part of the gym's KIDFIT program. No judgement here, but shouldn't an 11-year-old be out riding an ACTUAL BIKE on a Sunday morning, you know, OUTSIDE? Okay so that sounded a lot like judgement.

The music.

I recognised it, sort of. It was like the 9os - Radiohead, Red Hot Chilli Peppers - had been smooshed with doof and what had come out was something you would only listen to if your feet were strapped to a heavy device, preventing you from fleeing.

The machines talk back.

And they judge you. They flash green when you're in the "right zone". They flash red when you're working too hard. They don't flash anything when you don't know how to use them and you're pretty much replicating a flat ride to the bottle shop for 45 minutes. Apparently.

Watch: The six types of people you'll inevitably find at the gym. (Post continues after video.)

What the hell are all those classes?

So I fancy some pilates. I used to love pilates. Pilates and spin and maybe a little boxing. That'll do me. So what the hell is F45? And T28? And Tabata 30? And who can I ask, because...

The staff are all 12.

And they're every bit as gorgeous as side-boob-selfie lady. They're beaming and upbeat and I feel like they're really impressed that I have made it back to the gym at my age - because they told me - but I don't want to make them feel uncomfortable by asking them was Tabata is. It might be intimate.

So. I am back at the gym. And relatively certain that, given time, I will acclimatise, and soon I will be trying a French braid, and my bike will be flashing and I might even work up the nerve to try a Tabata.

But for now... anyone want to come for a plod? I'll be wearing my man-sized Ethan's Bucks' Crawl T-shirt, circa 2007.

Do you feel comfortable at the gym? 

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