In an extract from her book Total High, Grania Willis recalls mixed emotions as she sets off from high camp on the final push to the summit of Everest

The months of training and preparation had all been geared towards this moment. All the work that other people had put in, all the support everyone had given to me was focused on getting me to the summit. Now the onus was on me to perform. It was a weighty burden. I suddenly felt terribly alone and terribly vulnerable.

Such thoughts weren't helping me get ready so I refocused on the present. I carefully unscrewed the regulator from the top of the oxygen canister I'd used for sleeping, saving it as a reserve for the way back down to camp III later. I hefted a fresh four-litre cylinder on to my lap, attached the regulator and then started to tighten it. The violent hiss of escaping oxygen made me jump and I hastily screwed the regulator tighter to stop the noise. After checking to see that the valve was registering a full cylinder I turned back to the business of dressing and trying to eat noodle soup at the same time.

It must have been well after 11.30pm by the time I finally crawled out of my tent, pulling on my hood as I stood up. As predicted by Russ (the expedition leader), I was late but, despite the urgency I could feel coursing through me, I still made sure not to leave crampon holes in the mats as I got out of the tent.

It was incredibly dark outside. There was no moon and the stars were also concealed by cloud. It was oddly unnerving to be setting off in such inky darkness, with only a thin shaft of light coming from my head-torch to illuminate my surroundings. The last time I had climbed like this was on Cho Oyu (the sixth highest mountain in the world) over eight months ago. I'd done plenty of climbing in the interim, but only in daylight hours. Now, in the loneliness of the night, I was already higher than the summit of Cho Oyu and I still had an unbelievably long way to go before the sun's rays would start to break up the night. And I was completely on my own.

Camp seemed to be depressingly deserted. The rest of the team must be well ahead by now, I thought to myself. Looking up towards the summit, I could see the bobbing lights of my fellow climbers. Although I knew the general direction that I was aiming for, I could see no sign of the fixed lines that would lead up to the Yellow Band, and everyone else was so far ahead that it was impossible to tell whether I could head straight after them or had to take a more circuitous route.

I struggled over the sloping rocky terrain, stumbling over my crampons and almost falling as I tried to head in a right-handed diagonal path towards what I assumed was the route. It seemed an infinity before the beam of my head torch finally picked up the blue line of a rope and I clipped into it gratefully, knowing now that at least I was on the right track.

I upped my pace slightly and my crampons crunched on the scree, occasionally skidding awkwardly off a large rock and causing me to lurch sideways to regain my balance. Obscured by my oxygen mask, I couldn't see my feet or what I was treading on. I felt clumsy and uncoordinated. And I was getting uncomfortably hot. I pulled my hood off and felt the chill night air striking through my balaclava, but the cold seemed to refocus my energy.

I'd no idea how long I was plodding across the shingly surface, but by now I was on a distinct path with a rocky handrail on my left and a drop to my right. I didn't want to think about how steep or how long the drop was and kept my eyes trained on the route ahead. On the odd occasion I did look to the right, the beam of my torch vanished into blackness dark as a pint of stout.

Some time later I saw two lights not far ahead. I was catching up on someone, but it was a while before I got close enough to realise that it was Fujibayashi (my Japanese team-mate) and his Sherpa, who were making tortuously slow progress up the mountain.

The pair stood aside to let me past and, shortly afterwards, Karsang (my Sherpa) caught up with me. Even though we didn't speak the companionship was comforting and I felt my confidence increasing. I remembered Kiwi climber Mark Whetu's words back in Kathmandu almost exactly eight months ago and reminded myself that this wasn't just my summit bid, this was going to be my summit day.

The first piece of steep terrain came at the Exit Cracks, a series of narrow gullies that put me on notice that there was going to be some real climbing ahead of me. It was slow and painstaking work finding footholds, but it was more fun than the tedious trudge on shallower terrain and I knew it also meant that there was at least some height gain, even if it was only marginal.

I knew that Green Boots, the name given to the only visible part of one of the numerous corpses that litter the mountain, lay somewhere near the Exit Cracks. Thankfully, I didn't see the remains of the Polish climber, but I was focusing my concentration so much on the climb I would only have seen anything that had appeared directly in front of me.

Much to my surprise Peg (my Canadian team-mate) joined me shortly afterwards. I had assumed she was the owner of one of the tiny lights way ahead, but she apparently had left camp even later than I had. Like me, she was having to work hard to keep going. "I'm exhausted," she said soon after joining me. "Me too," I agreed.

We seemed to be moving desperately slowly, but we were some distance on from the Exit Cracks when I heard Fujibayashi calling in to Russ on the radio to say that he was turning round. He was complaining of chest pains and exhaustion. It sounded like he'd made the right decision to abandon his summit attempt.

In the dark I could sense rather than see a sizeable rock buttress rearing up in front of me. The First Step is the initial one of three steps and the beginning of the serious climbing on the summit push. I watched as Karsang started on the bottom section. The battery in his head torch had died early on, so he was really only climbing by braille. Even though it was familiar territory to him, it can't have been easy. I tried to train the beam from my torch on to the rock face, but Karsang's own shadow meant it would only have been of marginal help. I hoped that at least it might cast some light on footholds, even if it didn't illuminate anything higher.

There was an awkward move early on and Karsang, obviously handicapped by an almost total lack of visibility, spent some time finding a suitable crevice for his crampons before he hauled himself up. Things seemed to get easier after that and, aware that I would be providing even less light to Karsang if I stayed too far behind, I started on up the face.

I manoeuvred myself into position and managed to get my left foot high enough to make the move I'd just seen Karsang execute. It was quite a stretch, but nothing like the one under the mantle shelf at Dalkey Quarry where my climbing buddy Shane Cleary had told me there was a perfect foothold next to my left ear. That had been at sea level, however, and I'd been climbing in rock shoes. Now, at over 8,300 metres, wearing ice boots and crampons and in the dark, this was a far more intimidating proposition.

As always, once I'd committed myself to the move, it was easier than it initially appeared. Breathing heavily from the exertion I moved on up the rock to join Karsang at the top.

It had started to snow lightly and, as I looked skywards to check on the progress of my fellow Himex team members way above me, the flakes caught in the beam of light from my head torch. It was colder now and I pulled up my hood, tightening the drawstring to keep the snow off my face. Even though the bottom half of my face was covered by my oxygen mask, it was still too dark to wear goggles and the hood provided some protection for my eyes.

The terrain had levelled out and the flatter ground gave me a chance to recover from the climb up the First Step, but it also made for depressingly slow progress towards the summit itself. The snow was heavier now and I could sense that the traverse we were on was horribly exposed. I knew that the north-east ridge fell away down the Kangshung face to the left and the North Face to the right. I didn't want to think of the consequences of a fall here.

Peg had caught up with me again and stayed on my heels for a while, before asking if she could go past. She overtook me just before the traverse got seriously narrow, but she was moving at the same pace as me so we stayed together for some time.

It was distinctly colder by now and our slow pace didn't help, but my body temperature remained stable as long as I kept my hood up and the drawstring tightened. I knew once the sun rose it would warm up, but dawn was still a long way off.

We had been going for probably another half-hour when Peg suddenly stopped and said bluntly, "I can't go on." I was totally stunned and thought it was just a momentary lapse. Peg was one of the strongest and most experienced climbers of the entire team. She couldn't be contemplating turning round now. But she was deadly serious. "I'm exhausted. I can't go on," she said again.

"You can't stop," I said, "you've come this far, you've got to keep going."

I suggested we should have a drink and a carbohydrate gel to boost our flagging energy levels. She said nothing, but walked on for another 10 metres or so, before suddenly flopping down on to a large rock and then sitting there with her head in her hands. Even in the dim light of my head-torch I could see she looked totally shattered, both physically and mentally.

I took out my water bottle and handed it to her. I couldn't bear to watch her distress, so I turned away from the mountain, gazing into the darkness of infinity below me as I squeezed a sachet of carbohydrate gel into my mouth. The sickly sweetness of the gel was cloying, but I knew its energy-giving properties would kick in rapidly. Peg passed the bottle back to me and I took a swig out of it. I didn't know whether she'd drunk from it or not, but she ate nothing, even when I offered her another sachet of gel.

Without looking at me, she mumbled almost under her breath, "I'm turning round," and started to cry quietly. I was appalled. She really was serious. I snatched the oxygen mask off my face, put my arm round her and told her she'd be fine, just to keep going for a little while longer until she settled back into her rhythm again.

She shook her head. I could see she had already resigned herself to the fact that she was going down. I looked up in the direction of the summit, still swathed in darkness, and could see the lights from our team-mates way in the distance. Following my cue, Peg looked up too, but that just seemed to confirm in her mind that she was going no farther.

"Talk to Russ," I suggested. She sat up slightly and pressed the button on the side of the radio, putting in a call to the Himex headquarters at the North Col. Without preamble Peg told Russ that she was thinking of turning round. "Where are you?" he queried. "Just above the First Step. How much farther is it to the summit, time-wise?" she asked.

"Probably at least another five hours from there," Russ said. "I can't make up your mind for you," he went on, but he must have known from the weary resignation in Peg's voice that her summit bid was over. There was a pause before Peg said simply, "I'm coming back down". She was crying harder now as she turned to look at her Sherpa. "I'm sorry," she said.

I tried to comfort her, but she was inconsolable. It was her second time on Everest and still the summit eluded her. Would she ever get a third chance? Did she even want a third chance? I could almost see the questions whirling around in her head.

And there were plenty of questions whirling round in my head too. Self-doubt began to creep in. Was I right to continue if someone as tough and experienced as Peg was turning round? After the problems with her oxygen mask on the south side two years ago, did she know something I didn't?

I wanted to stay with her to try and console her. I knew she was shattered, but if I stayed I was going to find it very hard to get going again.

I was within a heartbeat of telling her that I'd come back down with her, but then I remembered why I was doing this. I wanted to climb the mountain for me, of course, but more importantly I wanted to climb it for Joe (my nephew who had died five weeks before I left Dublin) and for the two charities (the Irish Hospice Foundation and the Friends of St Luke's Hospital) that I'd pledged to support. I had to keep going.

I told Peg that I would have to leave her, but I'm not sure it registered. I kissed her on the top of the head and went on.

I didn't dare look back. I knew the sight of her misery would draw me back to her side and I'd end up heading down the mountain. I couldn't afford to look round.

It was probably almost as tough a decision as the one Peg had just made. She had given the mountain her best shot. I was determined to do the same. I wasn't finished yet.