Through running, Darren Clarkson found a new and unexpected response to PTSD, and a way to face down his demons. Within a year he has gone from a painful 500m to days of over 50km - but the numbers are just a means to an end. "Running is a fine teacher" he says.

It's almost a year since I lay prone, cold, on my lounge floor. Silent tears of failure filled my eyes and rolled down my face. Time passed like an echo of all the truth ever told. I don't remember getting in my car. I don't remember turning the key. I don't remember driving down the coast road. I do remember pulling into a dirt layby as a police car passed. I remember my body was shaking. I remember I was struggling to breathe. In time I must have pulled out into the road again. As midnight passed I found myself at a picnic site near a lake. The night was dark, still, painful. I left my car and went to sit on a bench by the lake. I felt nothing. I could smell the stains on my T-shirt and the urine on my shorts. In my hand I held a rope I use to tie kayaks to my roof rack. I do not remember picking it from my car. My eyes strained in the darkness at the branches overhead. How easy it would be to cast the rope over and take this void away.

My last 20 years have been about pushing myself, in the extreme, kayaking wild Himalayan rivers alone. These miles underfoot are just so very different, so much more. I have no words for the place they take me

I don't know what pulled me away that day, what made me return home in the darkness and early hours just before dawn. Over the next few days, I sat in a daze. Pain and confusion, one day rolling into the next. I couldn't eat; I couldn't sleep. Then one day I went outside, around lunchtime, my mind telling me I should run away from it all.

I am no runner, never have been. But my mind said left foot, right foot. I felt blood race around my body, the feeling my breath made as it escaped my mouth. I had to run. It felt like I needed to. But within 200m, possibly 500m, I was curled on the ground. Now in a foetal position, my head pounding harder then I could possibly predict, cold sweat making me clammy, I could feel the demon I had been running from hover over me. Walking home, defeated.

The next day I tried again. I would walk 100m then run 100m. Within minutes I felt the sticky salt tears roll from my eyes. Life sucked from me, I collapsed again. In time I walked back. On day three I pushed through the pain and the tears. I pushed harder and longer, just a kilometre, just that single lonesome kilometre. I sat and saw I had made it. It wasn't much at all – but it was mine.

The days kept coming. I kept going out and pushing past the pain and the hurt. I kept running from the demon, faster and harder than I could ever have known. At the end of the lane a track snaked into the woods, broken twigs and moss. A little more, a little time later, I went down slow walking now and again. Under the canopy of trees streams of light cast shadows and a maze of trails went off in all directions.

Still the mind, just made for the moment. My legs just kept going. My heart wasn't pounding out of my chest. The demon in whatever guise it had been before, was left far back on the tarmac outside the back door. At each twist and turn I just kept the pace. Streams crossed the path and I got wet feet – not that it mattered. Lost in time and pace. Lost to the motion and all that it was. I am sure birds called from the trees; I never heard them.

Once home with tight calf muscles and a coffee in my hand I realised I could enjoy this. As the sun came again the following day - left foot, right foot; I ran again and again. Days became weeks and each kilometre built on the last: 5, 10, 15 became just numbers on a map. Numbers that I tried not to fix on, numbers that really didn't make much sense in my concept of distance. I just ran from the pain. Then one day when my legs stopped I had made over 22km in a single hit. I had to Google why that mattered (it's a half marathon, if that even matters).

My last 20 years have been about pushing myself, in the extreme, on expedition, kayaking challenging wild Himalayan rivers alone. These miles underfoot are just so very different, so much more, so much so that I have no words for the place they take me.

I call myself a runner now – not just a guy who runs. I don't have a nutrition plan. I don't have a training plan. I run as much barefoot as not. Has the demon gone? No. Does the pressure of the world come crashing down time and again? Yes. But I know just through looking at things one step at a time and giving that process space, that the end will come. Perhaps not in an easy way; perhaps in a way that requires pain and suffering. The end will come. I am OK with that. Running is a fine teacher.