As I danced at my silver wedding anniversary party this summer, I couldn't have been happier. I was in the arms of the man of my dreams, feeling my usual spark of excitement as I felt his hand, unseen, slide under my breast, just for a second. 'You look gorgeous,' he murmured into my ear. 'Who could believe you've been married for 25 years.'

Married, yes, but not to him. I was dancing with my lover, in full sight of his wife and kids, and my husband and son – and no one suspected a thing. After ten years of having an affair, we know how to do this. Despite our brazen behaviour, we don't want to hurt anyone and have zero intention of breaking up our marriages.

Yet very occasionally we take a risk so insane that thinking about it afterwards makes me shiver. I know there is no excuse for what I am doing. At 52, we are old enough to understand the implications. But an affair doesn't have to be like it is on TV; all boiling bunnies, confrontations and drama. And I can't recommend more highly the almost miraculous effect an affair can have on a stale, middle-aged marriage.

My sanguine attitude is partly because I'm confident that neither my husband nor my lover's wife will ever suspect us. In the wife's case, as a size eight athlete obsessed with the perils of ageing, she would only regard a woman as willowy, fit and sinewy as a potential threat. In her mind, there's no way her husband would ever cheat on her with a shorter, rounder, older woman with big boobs and a wobbly bum, like me.

As for my husband, his ignorance is blissful because my infidelity has actually improved our marriage. While so many other couples in our social circle have grown apart as affection and passion have waned, these days we're happier and stronger than ever – giving him no reason to suspect things are anything other than rosy.

Yet ten years ago, pre-affair, it was so different. I was in the doldrums.

I wasn't unhappy, per se. My husband and I, who had been together for 15 years at this point, had a perfectly good life, regular good sex, a tolerably good work-life balance, and a gratifyingly cosy Three Musketeers relationship with our then 13-year-old only child. Living in Edinburgh, twice a month my work for a travel company sees me commute for two days to the office in London, rotating between staying with three single girlfriends. I've always had fun on those nights but, like my life at home, even they were becoming a little repetitive. Everything felt stagnant. I was becalmed and longed for excitement; something, anything, to spice up the pleasant but increasingly vanilla monotony of my life.

So you could say I was ripe for the plucking when, just after my 43rd birthday in May 2014, an old university friend came to stay.

We'd been close friends during our studies, but after graduating he'd moved to the States, married an American and largely fallen out of touch. Yet on the occasions we did exchange messages, things were still very chatty and easy between us.

So after he and his family moved to the UK it was a surprise, but not an unreasonable one, when he called to say he was visiting Edinburgh for work and could he stay for the night rather than booking a hotel?

Thanks to social media I knew he'd only got better looking with age but, even so, it was a shock when I saw him in the silver fox flesh. From the moment he kissed my cheek hello on the doorstep, my whole body crackled with electricity.

My husband was running late at work and my son was on a school trip, so we were alone for the first few hours. I hadn't felt this combination of molten lust and almost vertiginous giddiness since the early days of dating my husband.

Yet he seemed entirely relaxed and unaware of the maelstrom of sensations he had unleashed in me, and eventually I pulled myself together. By the time my husband joined us, we were chatting and laughing like the old friends we were, and the three of us stayed up late, putting the world to rights over a few bottles of crisp Picpoul. 'What a great guy!' my husband remarked, as we finally stumbled to bed.

We both left for work before our guest got up the next morning so, as I parked outside the office, I texted him instructions on where to find tea and coffee. 'Am nursing a sore head, but it was worth it,' came the reply. 'An even better pick-me-up than coffee would be the barista of the house delivering it to me in bed…'

I gaped at the message in shock. Was I the barista or was he just making a joke? Was it because I fancied him rotten that I was reading a sexual invitation into it? 'Is the barista of the house a 25-year-old figment of your imagination or is she mid-40s, slightly hungover herself and has her skirt buttoned up wrong?' I typed with shaking fingers. 'Why don't you come back and find out?' he responded. 'Oh, and I can help with the skirt.'

And that was that. I don't remember having a thought in my head as I turned the car around and headed back, but I know we never did have that coffee and I only made it back to work in the afternoon. Afterwards, I felt no regrets, no shame, just a thud of excitement whenever I remembered what we'd done.

I thought it would be a one-off but for the next few days, we texted like crazy. He immediately asked when we could meet again and we arranged to spend the evening together on my trip to London the following week. I suggested getting an Airbnb instead of staying with my girlfriends. He said that sounded like a plan with legs – great legs.

Our second meeting felt even naughtier for being planned – and the sex was even better for not being so rushed. Afterwards, he confessed that he too had idly kept track of me online over the years. Shortly after he moved back to the UK he'd seen a selfie I had recently posted on Instagram and felt a jolt of lust.

When he was commissioned to come to Edinburgh just a couple of months later, he thought it was a serendipitous moment to see whether I lived up to my picture. As fate continued to play into our hands – my job bringing me to London, his wife's job as an early morning personal trainer meaning they rarely socialised together on week nights anyway – it became apparent that this arrangement suited both of us.

About four 'dates' in, I asked how this was going to work and he came up with some rules of engagement: our marriages were sacred no-go areas, not up for discussion and not to be ended; total honesty with no taboos, both sexually and in our dealings with each other; no one was to get hurt, including our families. If either one of us stopped having fun, we'd end it.

And we had a lot of fun, because it turned out we both wanted to be as naughty as each other. I took the opportunity to play out all my secret fantasies, from wearing a blindfold in bed to passionately kissing in a dark alley.

It was all stuff I might have done with my husband and yet now the heady early days of our relationship were long behind us, it would have felt inappropriate.

I felt that if I had suggested to him we spiced things up I would have been expressing disloyalty to the sex life we did have.

Yet the day came, ten months into our affair, when I felt I might be falling in love with him. I confessed as much. 'Ah, now that's the last rule,' he said ruefully. 'No one is to fall in love. That's where it all goes wrong. Trust me.'

That's when I realised that, unlike me, he'd done this before: a six-year affair back in America that he'd ended when she got a bit too serious. That was why he was so upfront with me about the nature of our relationship, was careful not to see me too often –only about once every other month – and had so many tips about the craft of an affair: the deleting of messages, never calling each other unprompted, giving each other fake names in our contacts.

Hearing him talk, I realised I wasn't really falling in love with him – more with the excitement of the affair – and that I still loved my husband very, very much.

In fact, my affair has saved my marriage. For the past ten years, I haven't hankered for more 'magic' between my husband and me because, now I'm being indulged elsewhere, what we have is now enough for me.

No one knows me as well as my husband does, or gets my moods, my humour, my way of doing things, like him. And my love for him is no longer spiked with the everyday irritation I used to feel.

Money in our family has been tight the last few years, but these days I'm happy having baked beans on toast in front of House Of The Dragon with my husband, because I get all the wining, dining and illicit glamour I could want from my lover.

And not for me the invisibility of the over-50 female; being seduced by him has given me the confidence to go for new promotions at work. I am the envy of my friends for being so self-assured and sassy, for dressing sexily and not fading into a menopausal twilight of elasticated waists, for being nice to my husband.

In fact, as more of our friends have decoupled into divorce, we are renowned for the stability and love of our marriage.

Best of all, I'm no longer remotely in love with my lover. Over the years, he and his wife have become family friends: my son shadowed him for work experience, we've shared dinners out and been to stay at their country house. I'd be lying if I haven't felt uncomfortable on some of these visits, but it's not guilt – it's seeing him in a domestic setting.

Not surprisingly, he's different when he's not seducing me. He's unattractively tight with money, is boring about health and fitness and I don't like how he is with his wife when he's being the family patriarch – a little bit bossy and condescending in a way he would never dare to be with me.

He also keeps up the illusion with her that he doesn't really drink, which makes me laugh, as red wine is one of the things we enjoy best together. The pattern of an evening will be that we meet for a cocktail, go back to my Airbnb, strip off and have urgent sex. Then we'll loll naked on the sofa, drink a bottle of wine and, slightly fuzzily and languorously, make love again, more tenderly.

He is always the first person to look at his watch and say he has to leave. We've never fallen asleep together. It's the classic way to get caught, he said. And anyway, wouldn't it be more fun to keep it sexy between us, without popping the bubble with snoring and a 50-plus man's need to pee five times in the night?

I worry sometimes about how long it can continue. He's increasingly busy with work these days and I have considered that he could have his head turned by someone else.

I certainly haven't been tempted by anyone else.

I'm still deliciously happy with our arrangement. Without it, who knows what the state of my marriage would be?

Marina Ripon is a pseudonym