“You’ve got to step hard here” yells Alejandro. “This bit is really icy”. I push my foot forward with all the strength I can muster, but the crampons bounce weakly off the ice. I stumble towards a patch of snow where the crampons sink in, and grab onto a rock, breathing heavily. Alejandro looks back with a frustrated expression. “No, step hard!” he calls. Not for the first time, I wonder what I'm doing on this mountain.

On arrival in the city of Arequipa in southern Peru, many travellers are struck by their guidebook's claim that the nearby peaks of Misti (5,825 metres) and Chachani (6,075 metres) are “not technical” and are “some of the easiest of their height in the world”.

In New Zealand, to even look at a mountain you have to be able to tie 27 different knots, pitch a tent blindfolded, and know the meaning of words such as “carabina” and “belay”. As an exception to the generalisation that males are good with spatial tasks, I normally stick to non-technical recreations where I can’t directly endanger anyone else.

But everyone assures me that even I can tackle Chachani. “It really isn’t technical” says mountain climbing specialist Ulises. “Most important is that you have a strong organism, to handle the altitude”.

Two days later, at 5,200-metre Chachani Base Camp, I discover what happens when one’s organism falls victim to soroche, the debilitating sickness produced by high altitudes. My two fit-looking Israeli companions are prostrate in their tent with headaches, stomach cramps and nausea. After the gentle ascent to the campsite from where the 4WD has dropped us off, they’re struck down as our guide Alejandro leads us on a practice climb to the first pass.

Fortunate to feel only slightly dizzy, I’ll be one of the three out of nine people at base camp to attempt the summit. At 5,200 metres it's almost impossible for the unaccustomed to sleep, so I huddle in my sleeping bag until 3:00 am, when Alejandro and I drink some coca tea and set off, our headlamps lighting the way between the boulders.

At the top of the first pass, snow gleams in the moonlight. We have to don crampons to cross the face of Chachani's neighbour El Angel, and Alejandro warns me to "step hard" as each stretch of ice approaches.

Somehow I manage to stay on my feet, and after an interminable hour we’ve crossed the ice cap. "It’s better to do this by night” chuckles Alejandro. “You can only see directly in front of you. By day you can look down the slope to where you might fall, and you start to get afraid".

A long, cramponless climb up Chachani’s immediate neighbour, Fatima, follows, and here the altitude starts to take its toll. Time and again I stop, leaning on my ice pick and gasping, waiting for Alejandro to turn around so I can wave at him and yell "descanso!"

The sky gets light, revealing the flanks of the surrounding mountains, like huge beasts crouching in the dawn. Though the sunrise is spectacular, I’m absorbed in my personal battle with the loose, shaly surface of the mountainside. Every second step forward is matched by a half-step slide backwards.

Straining for precious oxygen, we finally come to the end of the steepest part of the ascent, cross a flat stretch of snow across the front face of Fatima, and ditch our crampons and packs. The final climb to the summit of Chachani itself will be made unloaded, carrying only my camera.

The purported “40 minutes more” seem like 40 hours. Just when it seems that the succession of false ridges will never end, Alejandro stops and waves back at me. I drag myself to the top of the rise and see him standing beside a simple metal cross. At last, the summit! The landscape is like a 360-degree topographical map; incredibly, we’re looking down on 5,800-metre El Misti’s volcanic cone, and the other smaller mountains and hills are like folds in a ruffled blanket. I don’t feel triumphant though; I feel half-dead.

On the horizon Alejandro points out 6,300-metre Nevado Ampato, where the famous "mummy Juanita" was found in 1995, intact and frozen after being sacrificed to the mountain gods 500 years previously. Thirteen year-old Inca princess Juanita and her entourage trudged up to the summit of Ampato in sandals and without equipment, before she was fed coca and alcohol and whacked on the head with a small mallet to speed her journey to join the gods. It wouldn't have seemed like such a bad option, I reflect.

And we still have to go back. I've expended every ounce of energy in getting to the summit, and the next three hours are probably the most physically difficult of my life. It’s the return across Angel’s ice face, slightly uphill, which is the real killer. Every step is pure, glittering pain. On the ascent I wasn't stepping quite as strongly as Alejandro ordered, but now my paces are like those of a drunken duck, crampons slipping and ice pick bouncing weakly off the frozen surface.

A couple of times I stumble; the spikes of the crampons catch and rip my trouser legs and I'm clinging precariously to a nearby rock At one point Alejandro has to physically help me over a tricky bit of frozen earth.

Long after my physical strength has gone we come to a broad flat stretch of snow - the last part of Angel's ice cap, beyond which is the descent to base camp. Dirt and boulders have never looked so beautiful. Painstakingly I pick my way back down to the campsite and collapse in the sun, joining the still extremely unwell Israelis.

On the road back to Arequipa, I enter a hallucinatory state brought on by exhaustion and dehydration; the rocky forms of the desert mould themselves into animals and human faces. I give myself up to it, and within two hours we're back in Arequipa. The air seems unbelievably warm and sweet; a hot shower like manna from heaven. I swear never to go near another mountain again.

After a couple of days of a dizzying head cold and nosebleeds, I'm feeling comfortable enough to sit on the terrace of the Casa la Reyna hostel in central Arequipa and admire the view of Chachani's triptych of peaks glistening in the morning sun. "I climbed that mountain" I boast to my French room mate as we share a cigarette. "Look, I'll show you the route we took"