Take a trip down memory lane and remember what you used to get up to during lunchtime at primary school.
If you were anything like me, you probably spent the entire hour running around playing games like "British Bulldog" and "Run Outs" with your friends.
As adults, just thinking about that level of exercise is enough make most of us feel tired. But as children, we barely even noticed our heart rates rising.
To help adults rediscover that endless feeling of energy, Charlotte Roach and Rosemary Pringle created Rabble - the fitness class that uses playground-inspired games to up your stamina.
While it may seem ludicrous to pay an instructor £14 to teach a game you used to play for free as a kid, Rabble is the perfect antidote for anyone who's sick of gruelling spin and HIIT classes at the gym.
The sessions are all about having fun and making new friends. Getting fit is just a bonus.
A former international athlete, Charlotte came up with the concept of Rabble after a near-fatal accident ended her career early and she realised how boring exercise for the general public can be.
"It was clear to me that this is why most people don't exercise as much as they should, I found myself dreading going to the gym," she says.
Players at Rabble are split into teams that last for the entire one-hour session. The core concept is that you're so focussed on winning that you get fit without realising it.
"After a few weeks people get much quicker, even people who run regularly notice their increase in speed," Charlotte says.
"Lots of our games are disguised high intensity intervals because this is the most efficient way of getting fit and burning fat. We also naturally train agility and coordination, which are two areas regularly neglected in fitness after school PE."
Rabble run classes every day of the week at various London and Manchester locations, but I joined a group on Kings Cross basketball courts, led by instructor Josh Kingcott.
After a quick warm up of tag, the 10 of us at the session were split into two teams.
The aim of the first game was to throw a frisbee five times between your team members without the other team intercepting. After a few rounds, the stakes were raised and a goal was introduced for extra points.
It wasn't long before my long-forgotten competitive streak resurfaced. I found myself shouting "mark up!" and "free if you need!" like I was back in the year 10 netball team.
I got so carried away that I even barged into one of my fellow Rabblers for a dirty intercept (again, I am very, very sorry about that). Thankfully all was forgiven, because at Rabble everyone laughs as much as they run.
For game two Josh marked out three circles on the ground using cones, which were each worth different amounts of points. The attackers had to sprint to the circles while the defenders had to try to tag them "out".
It's been a long time since I've felt the need to sprint anywhere and it wasn't long before I was absolutely exhausted.
I'm pretty sure the people in the nearby pub could hear me panting, but I couldn't give up and let my team down. My sprint decreased to a jog and occasionally a walk, but no one seemed to mind.
As I looked around at my teammates they had equally flushed cheeks, despite the cold weather.
The 10 people at my session clearly had varying levels of fitness but it didn't affect the camaraderie. Everyone pushed themselves as hard as they could in order to do their best for the team, and you can't really say fairer than that.
After all that running, playing dodgeball while confined to a small circle came as a welcome respite.
I didn't feel the need to look at my watch during Rabble and the session was over before I knew it.
There was just about time to have a team photo under the floodlights (a Rabble tradition that occurs after every session) before I had to jump on the train home.
One of the best things about Rabble is the relaxed vibe. While I sometimes feel underdressed at London gyms because I don't own a pair of fancy Sweaty Betty leggings, I rocked up to Rabble in my old, warm university hoody - and nobody batted an eyelid.
The group I joined run a supper club every other week where they grab dinner together at a local restaurant after the session.
I have no doubt that my fitness would improve if I attended Rabble every week and I also think there's a pretty strong chance I'd make nine new friends.
The team at Rabble also run themed events, such as 'Kiss Chase' for Valentine's Day and regular 'East Vs West' London battles.
But at £14 per 60-minute session, I was a little disappointed by the price of an average class. While it's cheaper than many exercise classes on offer in London I expected a concept with so few overheads to be more of a bargain.
That said, if you want to improve your stamina and meet an awesome group of people, I'd highly recommend playing. You'll feel like a kid again before you know it.
Find out more about Rabble or book a session at joinrabble.com.