As a 20-year-old student touring America during my summer holidays in 1989, life seemed endless and full of possibility. I had travelled by Greyhound bus from New York to California. Now I was spending three days walking in the High Sierra mountains; sleeping in a tent, heating corned beef on campfires and drinking from streams.

Most visitors to Yosemite don't venture far from their cars, so after walking for a day and a half I was totally alone. I was enjoying the silence as I descended from Clouds Rest, a peak of more than 9,000 feet, when I lost the path. As I tried to rejoin it, I walked through rough vegetation on a slope that was getting steeper and steeper. Then I stepped on to wet ground and found myself sliding down a rock face. I grabbed at greenery but accelerated until I hit some rocks and knocked myself out.

When I came to, my leg was bent under me so that my right foot was almost touching my head. Somehow I got into a more natural position, and only then did the full realisation of my situation hit me. My right femur was broken, nobody knew I was there, I had food for another day, I was in an isolated part of the park and was some way from the nearest footpath.

Like the moment after the door shuts and you realise the keys are locked inside, my mind raced around every possible escape route, but found no answers. My leg throbbed and ached. Whenever I moved I could see the skin stretch as the bones shifted underneath, and there were sharp, agonising pains that made me cry out.

I tried to crawl back the way I had come, but my weakened state made this impossible. The route was steep and I dropped my water bottle, which rolled away. As I crawled after it I disturbed a hornets' nest and was stung repeatedly.

I shouted for hours that first day, hoping the occasional person that I had passed on the trail would be close enough to hear me. As night fell, I put up my tent against a tree to stop me slipping down the hillside.

Over the next few days I developed a routine. During the day I crawled on to a rock in the sun to try to signal with my red sleeping bag. There was an amazing view of Half Dome mountain and row after row of distant ranges. If this was where I was to die, it was certainly beautiful. Every minute or so I shouted, giving my location and situation. I lit fires to try to attract planes. I also had a mirror that I used to reflect the sun, but this didn't work like in the movies.

One day I felt something behind me. I turned and saw a black bear running away - he must have been snuffling in my bag before I disturbed him. I knew that black bears were not threatening to people, but my heart raced.

As the days passed I shouted for help less and began to give up hope. The 14th night was the first when I didn't have nightmares. I dreamed of friends and family, and when I awoke, I felt calm. I looked down at my wasting body and realised that if I did nothing, I would be a skeleton before long. I ate the last of my food, filled my reclaimed water bottle and set off up the rock face.

The rocks grazed my legs as I dragged myself backwards over them. At the steepest section I was able to jam my hands into a crevice to hold me as I manoeuvred my good leg to keep pushing me up. A rush of adrenaline got me over and I knew then that I'd make it. I slid back two feet for every three I climbed, but after seven hours I was back on the path.

Two hours later a couple wandered past. Initially they didn't realise I was injured and we exchanged pleasantries for an excruciating few seconds before I abandoned my English reserve and blurted out my story. They left me some chocolate and went for help.

Eight hours later, in the dark, two rangers appeared. They put my leg in a splint and called in a helicopter for first light. I spent two weeks in hospital in America and was back home for the autumn term.

In the 18 years since this happened, those 14 days have become a touchstone to remind me what a privilege it is to be alive. We all know that our time here is temporary and that it can end in a second. With this experience I was reminded of my mortality for two long weeks, and I never feel very far from this knowledge.

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