DNF stands for ‘did not finish’ – the acronym no ultrarunner wants assigned to their race result. For amateurs, DNFing is loaded with emotion – but attrition rates are extremely common

Drenched, freezing, utterly miserable and with a GI catastrophe ready to blow, my internal conflict of quit/don’t quit was in full force. I was at about kilometre 56 of my 100km ultramarathon. After a cascade of bad decisions and worse weather, totally wretched, I shuffled and sobbed my way through the dark to the next checkpoint. By the time I got there at 62km, the resignation of dropping out had settled in. Months and months of training had come to nothing.

Inside the checkpoint, soaked runners and their besieged crew were crowded into what resembled a field hospital triage unit. In the chaotic throng I found my crew, slumped down on to a bench and said: I’m pulling out.

David ‘Jacko’ Jackson has aimed to raise awareness of brain injuries, with fellow runners sharing their stories

The key for David “Jacko” Jackson is to take each stage nice and gently. “It’s not about running fast,” he said. “It’s more like the hare and tortoise story. Slow and steady – and do it again tomorrow.”

So far, so good. Jackson, a former professional rugby player who retired after a traumatic brain injury, is nearing the end of an extraordinary bid to run 100 ultramarathons (50km, or about 30 miles) in 100 consecutive days.