The back straight on the third lap of a mile track race.

The fourth lap at Parliament Hill.

The ninth kilometre of ten.

The final miles of your first marathon.

The seventh hour of a Bob Graham Round.

The fifteenth hour of a Ramsay’s Round.

These are the tipping point moments of running when relative time, distance or experience is irrelevant: suffering has begun.

The latest of these came on Sunday lunchtime, at the two-and-a-half-hour point of a Pentland Skyline. I reached the boggy cleugh between Black Hill and Bell’s Hill, and as the adrenaline and concentration of several minutes of swift descent subsided, it was replaced by a thump of nauseous exhaustion. I felt suddenly as if I hadn’t slept for a month (which is partly true). Bell’s Hill is the lowest summit of the 17 on the Skyline circuit, yet it is approached via a one-in-three grass ramp that reduces all but the very best to a pained hands-on-thighs shuffle. Quite naturally, I want to stop. I want to sleep. Those runners who I see in a glance downhill, do they feel what I feel? And yet I also want to fight. It is the day after Eliud Kipchoge ‘broke 2’; today, here, in these misty, damp hills, in shoes stuck together by glue, I just want to break 3. We all have our limits.

These are the two essential juxtaposing ingredients of suffering – exhaustion and perseverance. Because if you didn’t persevere, you wouldn’t be suffering.

I have felt like this before, of course. I have run three mountain rounds of around 20 hours, but if I had to rank order suffering, top of the list would be a mile race at Tooting Bec. I wanted to at least break five minutes for the mile – it seemed the benchmark for a ‘good’ club runner. On the back straight, around 950 metres in, I was leading – and suffering. ‘Swimming in lactic’ was the term a clubmate later used to describe this bout of running-induced distress. One step is all it would have taken. One step to the left, over the metal bar on to the inside of the track, and the race – and the suffering – would have been over. No-one but me would have cared. But I persevered. It was electronic timing that day. No stopwatches, no excuses. My time was 63 hundredths of a second outside five minutes. It will probably be a lifetime personal best. I haven’t run a mile race since.

In the mountains, on a 24-hour round, running through the night, the suffering is different: it is duller and deeper. Suffering on the track or road is the feeling of suddenly being immersed in cold water – shocking and stark. You can jump out soon though. Suffering in the mountains is when you have finally got used to the chill, but then, inevitably, you cool down and start to shiver uncontrollably, and you feel you could just slip silently into the water.

The question is, I suppose, why. When Runner’s World asked me this question recently this is what I wrote in response:

‘Why are you going running again?’ my six-year-old daughter asked.

The emphasis was on the word ‘again’, as if she was inwardly frowning at some sordid habit of mine. I stared at her open-mouthed. In the moment, I could think of no response.

We were on holiday in Annecy at the time and I was reading Murakami’s Men Without Women. What I Think About When I Think About Running might have been a more appropriate searching place, but the answer to my daughter’s question came quite unexpectedly.

Kitaru was explaining why he wanted his beautiful girlfriend to date other people.

‘It’s like, we graduate from college, get married, we’re this wonderful married couple everybody’s happy about, we have the typical two kids, put ‘em in the good old Denenchofu elementary school, go out on the Tama River banks on Sunday. “Ob-la-di, ob-la-da”… I’m not saying that kinda life’s bad. But I wonder, ‘y’know, if life should really be that easy, that comfortable.’

It was beyond me to explain Murakami’s notion to a six-year-old, but this was why I was going running again. Because, in 25C heat, I wanted to climb the 1,300-metre mountain that stands over Lake Annecy. Because I wanted to leave my life of comfort and control – air conditioning, food and drink, the security of my family – for a little time, for a little discomfort. And that is why, I suppose, I run again and again: because, as Kitaru asked, should life really be too ‘easy’, too ‘comfortable’?

But here is the irony: in running, discomfort brings tremendous comfort. My last race was Philiphaugh Hill Run in the Scottish Borders, seven miles of rough, wild moorland. It rained incessantly. Paths were gushing streams. The world seemed in tears. There was nowhere I would have rather been: standing at the very pinnacle of uncomfortable comfort.

To suffer through running is not my aim, but it is a consequence. As Murakami suggests through the characterisation of Kitaru, what is life – or indeed what is running – without some discomfort? Even so, when suffering comes and however it comes, I don’t necessarily embrace it. I am not a masochist. The purity of that moment becomes no less startling however often it has been experienced.

But that is why this is an art – to see (and to feel) such suffering is beautiful. It has raw emotional power. One of the anecdotes I tell in The Mountains are Calling is of running up the very same Bell’s Hill with Mick James who was approaching the end of a 50-hill, 60-mile circuit of peaks south of Edinburgh to mark his 50th birthday. It was mid-June, but night had fallen and we were guided by torchlight; Mick moved as if his legs were constructed from wood. ‘You know when you see refugees who have walked hundreds of miles just in the clothes they are wearing,’ he told me. ‘The reason they can do that is because humans can. Of course, you must be so far out of your comfort zone to have that experience. This is like a first-world version of that.’

Suffering teaches us what we can do, physically but also emotionally. Anyone who has watched bone-weary, near-delirious runners in the final six miles of a major city marathon can tell you that: epiphanies are positively lurching up and down the road. Ultimately, running is a sport at its most beautiful when at its ugliest.