People run for different reasons. Health, appearance, social opportunities, the temporary postponement of death, are all fairly standard ones. What suddenly propelled me into a life of running was the activities of some particularly rapacious canines, who would eat my house and all of my possessions were they were not kept continually exhausted. Enjoyment is also a common reason – I have my moments, although perhaps not as many as I used to. So, running has a point. Many of them, in fact. But, ironically, all talk of the point of running actually misses the point.

We usually understand the value of an activity in terms of its purpose. The value of running, we might think, lies in the health it promotes, the sleekness it restores, the pleasure it brings. This would mean that running does not have value in itself. Money and medicine have what is known as instrumental value – value as a means only. It is not the piece of paper or the pill that has value, but the things you can buy with it or the health it can restore. If the value of running is simply its point, then running, too, has only instrumental value. The question is whether this is the only worth it has.

Imagine a utopia, where all the usual reasons for running don't apply. The multifarious purposes of running – health, physical fitness, sense of personal satisfaction – can be achieved by taking a pill. Would you still run? If your answer is yes, then you must think that running has something more than mere instrumental value.

Yes, would be my answer. I suspect it is connected to something that happens, on longer runs especially, when I become lost in the run's beating heart. Of steps and breath, counting out time elapsed and distance covered. There, I am reminded of something I used to know, once upon a time: in the end, points and purposes don't really matter. In this moment I cannot make myself care about these things. I am running simply to run.

This experience is found in other sports too: an absorption in the deed and not the goal; the activity and not the outcome. This is play in its purest form.

There are things we do for their own sake and things we do for the sake of something else. The latter we might call work – for this is the classic example of an activity done for the sake of something else. The things we do for their own sake – that we do just to do them – are, correspondingly, forms of play. To assume that the value of running lies in its point is to assimilate running to the category of work. But the real value of running is that it is not work but play.

Work has instrumental value. A life composed entirely of work – of activities we do for the sake of something else – is a life where value is always just out of reach. Suppose you do A only to get B, and B only to get C, and so on. This is a life spent continually chasing, but never catching, what is important or valuable.

There is, however, another possibility. Eventually A, B and C lead to something – call it Z – that is valuable for what it is in itself and not for anything else it might get you. This sort of value is known as intrinsic value, and it has a long history. Plato called it The Good. You experience The Good when you are aware of what is valuable for its own sake and not for other things it might get you. This is precisely the experience of giving yourself over entirely to the deed and not the goal, the activity and not the outcome.

The experience of play is the experience of intrinsic value: playing is the embodiment of The Good. It is The Good that I experience on a long run, when I reach that place where all the points and purposes of life stop and I realise that I am running just to run.

I suspect I used to know The Good much better. We think of childhood as a time of preparation for the important part of life that comes later. The child can't know what is important in life: it's only when we put aside childish things that we come to understand what is valuable and what is not. I suspect this gets things the wrong way around. Children understand intrinsic value – The Good – better than we do. They understand it instinctively, effortlessly. For us, it's hard work. Children know that what's worth doing in life is worth doing for its own sake. And everything else is merely unfortunate. This is something we forget when we begin the great game of growing up.

Running is a place where I remember what I used to know. That is not the point of running. It is what running is.

Running with the Pack: Thoughts From the Road on Meaning and Mortality is published on Thursday by Grant, £12.99. Mark is appearing a the School of Life in London on Tuesday and Run and Become in Edinburgh on 11 March