Exercise is usually a solitary activity for me. Although I like the idea of a workout buddy, since having children it has been impractical. As a working mother, opportunities to exercise are snatched in between deadlines and school runs and going to the trouble of coordinating diaries with one of my friends, just so we could put ourselves through a few squats and crunches felt like a waste of what could be a good night out.

Then a few months ago, I noticed that the gym where I’m a member had a scheme where children aged 11-plus could take part in workouts supervised by their parents. I wasn’t sure whether this would be my eldest son Otis’s cup of tea. Although naturally energetic and an enthusiastic member of the school rugby team, his experience of keeping fit and active was like most children’s – involving friends, spontaneous action and, judging by the state of his PE kit, more mud than Glastonbury.

Much as I enjoy the gym, exercising in the sanitised, air-conditioned environment I’d grown used to felt slightly joyless by comparison. I wasn’t sure it could compete. More to the point, I wasn’t sure I could compete. Unlike his rugby pals, I rarely discuss the intricacies of the new Xbox update.

Yet, with Otis’s 11th birthday looming, he wanted to have a go. I think working out in a gym carried the same mystique as all those other things that adults do but children can’t: driving a car, going to work, swearing without risk of being chastised. I was sure that it wouldn’t be half as interesting once he’d had a go. But I was wrong.

Before he was let loose on the gym floor, Otis had to have two sessions with a fitness instructor. I’ve no doubt that putting an eager 11-year-old at the helm of a Stairmaster without warning him not to start on level 20 has the potential to end badly.

By the time Otis was issued with a special wristband and unleashed into the gym with me as his chaperone, he was raring to go. And so was I.

Now that he has two younger brothers, aged seven and three, the two of us don’t often have the opportunity to spend time on our own. These gym visits offered a glimmer of those days when Otis was really small, when we could chat or play without risk of being constantly diverted.

We head to the gym on a Sunday now, for 40 minutes or so before lunch, so we’ve earned our apple crumble. While it’s not the toughest workout I do, it’s fulfilling for the simple reason that it’s about so much more than the exercise.

We jog side by side on the treadmill and discuss what’s been going on in his week. Occasionally we’ll drift on to what he’s seen on breakfast news about Tim Peake or the situation in Syria. More often, it’s which of his friends was given detention for playing football with a jelly in the canteen. Our conversations usually aren’t big, important ones, but that’s the beauty of them. They consist of the small and, on the face of it, insignificant trivia that is probably more important than I’d ever really given credit to.

There are, of course, certain things you have to come to terms with when you start working out in a gym with one of your children. Such as the fact that they only want to stay on any given machine for a maximum of three minutes, before getting bored and wanting to try something else. They will tell you that you have been doing something – be it squats, crunches or burpees – all wrong for the past 20 years because the instructor at his first session did it differently. They will also get irritated if you manage to do more press-ups than they can. But possibly not as irritated as you will, if it’s the other way round.

Against all my predictions, our workout sessions have gone from strength to strength, so much so that my father, who is 70, joins us on the odd occasion too.

Collectively, we must look like the stars of the world’s worst exercise video. But there’s something I love about the idea of three generations of our family having a regular get-together like this, shooting the breeze as we dawdle along on the elliptical machines.

I couldn’t claim that this experience has transformed our fitness levels – there’s a limit to what you can achieve in one short session a week. But the value of it has gone far beyond any cellulite I’ve managed to shift or steps I’ve added to my daily count. It has given me and my eldest son the opportunity to spend time together, doing something we both enjoy.

We could probably achieve that by doing any number of activities – climbing mountains, cycling, indoor sky-diving for that matter (and we’ve done all those on occasion, along with his brothers).

The difference is that this is easy, a regular set time out that doesn’t involve great expense or fuss and is only 10 minutes from home.

I don’t know if it will last for ever. But I’m happy to continue with the press-ups for as long as it does.

• Jane Costello’s new novel, Summer Nights at the Moonlight Hotel, is published by Simon & Schuster, £7.99