In 2003, I was working for a firm of accountants in Glasgow and training to become a Chartered Accountant. I failed my tax exam, and my mental health suffered. My employer’s policy was ‘two strikes and out’, so I knew that if I failed my tax exam again, I would lose my job. I had dropped out of university twice, and I had taken five years to finally complete my undergraduate degree.

I believed that failing my exam again and losing my job would be the end of not only my career, but my life. I began to spiral into insomnia, anxiety and depression.

Finally, I sought help from my GP and was prescribed sleep aids and antidepressants. I took a month off work to regain my perspective. When I was well enough to return to work, I re-sat my exams, and I failed my tax exam again, and I was made redundant. To avoid depression, I knew that I needed something really positive to focus on while I searched for another job, and so I took a major risk and entered a 150-mile ultramarathon in the Sahara Desert – the Marathon des Sables, known as the ‘toughest footrace on earth’.

The Marathon des Sables is a stage race, which takes place over seven days – it’s the equivalent of running seven marathons in seven days, although the stage lengths do vary. The longest stage is fifty miles, or eighty kilometres, and the shortest stage, on the final day, is a half-marathon. Daytime temperatures in the Sahara touched fifty degrees Celsius, and at night the temperature dropped to zero. Competitors carried all of their clothing, sleeping bag, stove and food.

As the external temperature increases, your body tries to dump excess heat to maintain a normal body temperature of 37°C; your heart rate increases to pump more blood to your skin, and you sweat.

Cardiovascular exercise creates additional metabolic heat and adds to the body’s heat burden. In very hot environments, the rate of "heat gain" is greater than the rate of "heat loss" and the body temperature begins to rise, which can result in heat illnesses, such as heat exhaustion or the more serious heat stroke.

Preparing your body for such a range of temperatures is daunting. Some runners train on a treadmill or stationary bike in a sauna. Hot yoga is also a modern practice that might have helped with acclimatisation to the extreme heat.

I had little time to train properly for the event as I was playing semi-professional rugby and the event took place in rugby season. The longest training run I completed was only thirteen miles, on a chilly Spring evening in Glasgow.

On day two, I suffered severely infected blisters. The medics sliced off the blisters, applied iodine to the raw flesh, and then Compeed and tape, which allowed me to continue. On day four – the long stage – I ran with two highly experienced runners from Italy. I over-extended myself, became severely dehydrated and suffered from heat exhaustion.

At forty miles, I began dry vomiting. At the checkpoint, the medics gave me rehydration salts, which I drank and then vomited straight back up. It was now essential for me now to replace the water and salts lost through excessive sweating, to prevent my condition from worsening to become potentially fatal heat stroke.

Since I was now unable to keep anything down, the only option left was intravenous fluids – I received three bags of fluid via IV and camped at the checkpoint. In the morning I finished the stage, and two days later I crossed the finish line, and experienced such an intense feeling of elation, and relief. I had pushed my physical and mental boundaries far beyond what I had believed to be possible.

Completing the event totally changed my mindset, and I turned my attention to the world’s high mountain ranges. I was no longer afraid of failure, and I would never again allow my own fears or self-doubt to hold me back.

Fourteen years and eighteen expeditions later, I was climbing high on the summit ridge of Mount Everest. I had turned back the year before at 7,900m – I had been unable to eat or sleep, and I chose not to enter the Death Zone above 8,000m because my energy and oxygen levels were too low.

At 7,000m, my oxygen saturation was measured at 52%, which is almost half of the sea level value. On the summit of Everest at 8,848m, the oxygen pressure is only 30% compared to sea level. Many climbers have entered the Death Zone and never returned – on the northeast ridge of Everest we passed many bodies as we climbed high on the ridge. To prepare for this very low oxygen level, we had spent six weeks slowly climbing up to the North Col at 7,000m and above, then descending back to base camp to rest.

These acclimatisation ‘rotations’ gave our bodies time to adapt to the lower oxygen levels by generating more red blood cells, to allow our bodies to make more efficient use of the reducing oxygen in the atmosphere. Our breathing rate also increased as we climbed higher - a shorter-term response to low oxygen levels. The downside of this increase in respiration was that we were losing a significant amount of water vapour in our breath and would easily become dehydrated. To compensate, we tried to drink at least five litres of fluid every day.

It was now 18th May 2018, I’d been climbing for the past eight hours through the night, and it was still dark. The temperature was twenty below zero. The winds were much stronger than forecast – so strong in fact that my guide had turned back four hours before. The eyelashes on my eyes were frozen together. I was alone on the ridge with my Sherpa, Phurba, and we were the highest team on the mountain. For the last four hours, we had been the highest human beings on earth.

I was wearing a down suit, triple high-altitude boots and high-altitude mitts. I carried two oxygen cylinders in my rucksack and I wore an oxygen mask on my face. I carried only one litre of water inside my down suit, and two energy gels. That is all I would eat or drink for the next seventeen hours. Every step required a monumental effort of will.

After an exhausting climb, we had passed the three main obstacles on the northeast ridge – the first, second and third steps. These steps are vertical boulders, up to 40m high. I could see a bright light far off up high to my right. I was sure this marked the summit, so I knew I still had another four hours of climbing ahead of me to climb as high as that light and reach the summit.

As I climbed the steep snow slope in the pre-dawn twilight, I saw a shape fluttering in the wind, fifty steps ahead. It looked like a prayer flag, which Sherpas place on the summit of mountains in Nepal and Tibet. But why would someone place a prayer flag so low down on the summit ridge?

The light of my head torch picked out a few more fluttering shapes, and I was hit with an overwhelming realisation. These were flags, and they were on the summit of Everest. For a moment, I stood absolutely still. I was in shock. I turned to Phurba:

“Phurba, this can’t be the summit.’

“Yes, summit.”

I was hit by a wave of euphoria and disbelief, and I fought to hold back my tears. The light I had been following was a bright star. I turned to embrace Phurba, here below the summit, and I thanked him sincerely for guiding me to this remarkable place - the highest point on earth.

Growing up on a council estate near Glasgow, it had seemed an impossible dream.

I stepped onto the summit of Everest at 06:15am, with Phurba one step behind. The wind was howling. We clipped in to the fixed rope and embraced again. I had achieved my lifelong dream.