“Halfway mate, keep it going.” But that can’t be right. We’ve been running up this spiral staircase of despair for at least a week already. My legs are on fire, my chest is about to cave in on itself and sweaty mess doesn’t come close to describing my state of perspiration. Sunday afternoons weren’t made for this.

Fortunately, there’s a very good reason for my being in such a world of pain. I’m taking part in Stand Up To Cancer’s London 3 Peaks Challenge: a once-in-a-lifetime chance to run up three of London’s tallest towers (The Gherkin, Salesforce Tower and 200 Aldersgate), abseil(?!) down the third and raise a load of cash for the fight against the disease.

Most participants have raised over £600. In fact, my stair-climbing brother-in-arms, Kyle, who I met at the start line, raised over £850 and is here in memory of his late granddad.

Back to the world of pain. Whoever built The Gherkin clearly didn’t do so with stair runners in mind. The steps are a hard-to-see dark green and the unventilated, windowless climb is stifling. Fortunately, the view from the top is as stunning as what came before it was painful. Better still, the official race timings get paused once you reach the top of each tower, allowing for some breath catching and the chance to fully appreciate the sprawling, aerial view of London.

Another plus point: returning to ground level is done so with the help of an elevator – a marvelous invention. Once at the bottom, it’s a short jog over to Salesforce, which a sympathetic looking security guard informs us is “a bit tougher” than the leg-numbing struggle we’ve endured.

It’s certainly higher, with 61 floors to The Gherkin’s 40, but the addition of windows all the way makes for a much less nauseating ascent. Nonetheless, 30 floors up and I’m walking, opting for a hand-on-thigh lunge up two steps at a time. Kyle, who I only met half an hour ago but who I feel I’ve known for ages, is somehow still soldiering on with a one-step-at-a-time jog. Volunteers at strategically placed floors offer motivational words, which helps – although one bloke simply said, with a grimace, “Rather you than me, pal.”

Finally reaching the top, we stumble into a huge area of chairs, foam rollers, a food stand and 360-degree views of the capital. Stunning.

Not so stunning, is the lift that we have to take back down. “I’m afraid it’s an industrial elevator,” says the cheery woman at the top, “so it’s a bit smelly.” Massive understatement. Nonetheless, after a brief exercise in vomit avoidance, we reach the bottom and make for the final tower. Much smaller than the previous two – hoorah! – but distinctly more terrifying.

The 30-something floors are a comparative doddle compared to The Gherkin and Salesforce, but this time the challenge – as Kyle and I duly discover on reaching the top – is getting down. Scores of weary, nervous people are being harnessed up in preparation for something that couldn’t be further removed from the comfort of a Sunday roast.

All too quickly, it’s my time to take to the edge. The chap tasked with attaching my ropes and, therefore, keeping me alive, does his best to conduct some friendly, this-is-no-big-deal small talk, bless him. But I’m having none of it.

I’d love to say I made the most of this unique opportunity; taking everything in and reflecting on the ridiculousness of the situation. In reality, one cursory, vertigo-inducing glance over my shoulder meant that I spent the duration of the descent in such a trembling state of concentration and fear that I got to the bottom and realised I hadn’t really registered what I’d been doing.

Clearly, though, that’s not the story family and friends will hear. In their minds, I shrugged off the instructor’s warning that it was too perilous, that only a madman would attempt such a feat, and glided to the floor. Think Tom Cruise in Mission Impossible 4.

And that was that! There’s no doubting the fact that London 3 Peaks is an almighty challenge (my legs, for days afterwards, can testify). As such, it’s a fitting tribute to the battle that millions of people all over the world are facing right now. Cancer is awful and indiscriminate, but it can also be beaten. I can think of few better ways to step and join the fight.

Register your interest in the 2016 event by emailing info@standuptocancer.org.uk

standuptocancer.org.uk/london3peaks

Isaac (left) and Kyle at the finish

Isaac (left) and Kyle at the finish