But things took a turn when, over post-run drinks one afternoon, he admitted something unexpected.
"You know, just because someone's married, doesn't mean they can't be attracted to someone else …" he began, the ice trembling in his glass in synchronicity with the stammer in his voice.
It was the initiation of what would go on to be increasingly clear and frequent declarations of interest in me.
"I'm consumed with thoughts of you," he texted one evening while away on holiday.
"I'm not happy in my marriage. I'm leaving her," came another confession.
This was what I wanted. I should have been exhilarated.
Instead, with the barriers to our union removed, I realised – much to my own surprise – I wasn't attracted to him at all.
As it turns out, lusting for someone simply because we either can't or shouldn't be with them is hardwired into our DNA. Science has a term for it: the "forbidden fruit effect".
Research indicates when something is off-limits, we're more likely to get hooked on it – whether that's food (think back to the last time you dieted and found yourself daydreaming about face-planting a bucket of KFC), an object (slap "waitlisted" on just about anything and most of us will hand over our credit card faster than you can say "impulse purchase"), or a person.
Meme culture acknowledges the power of this effect in relationships.
A viral tweet that reads: "People say there are plenty o' fish in the sea. Yeah, well I got my eye on that one specific emotionally distant salmon with commitment issues," and a TikTok trend that sees people dancing to 'Twerkulator' under the caption: "Are they actually hot or are they just ignoring you?" epitomise the conflicting desire to be wanted in the face of feeling turned off by people who like us.
British philosopher Alain de Botton dismantles this paradox in his iconic New York Times essay, 'You're Probably Married To The Wrong Person', proposing, "We aren't on a quest to be happy; we're on a quest to suffer in ways that feel familiar."
Grim as it sounds, Botton suggests if we didn't receive healthy love growing up, we'll be drawn to people who are unavailable as adults, and who ultimately reinforce a belief that, at our core, we're unlovable.
And while it's tempting to think once the hurdles are removed, we'll be ready to surrender our hearts, in reality, the reverse is true.
In fact, a paper reviewing infidelity research by Zur Institute found most affairs never make it beyond the "falling in love" phase, let alone transition into long-term committed relationships. Sexual attraction to an off-limits lover is so intense precisely because it can't be fully realised, not in spite of it.
The real challenge is in allowing ourselves to get close to someone with whom there are no obstacles; to readily let love in.
And if that scenario sparks repulsion or sexual apathy in you?
Botton counters this best when he says, "Love can be hard to receive when we're not convinced of our own lovability."
Admittedly, being able to recognise your innate worth is easier said than done. Even more than a decade on from my married crush, I'm only just beginning to embrace my own.
But while I'm still tempted to chase an elusive figure back down a well-worn path, these days I find myself far more excited to lace up and head down a less familiar track – with someone who runs toward me.
