Towards the end of the year before last, I began swimming every day at Tooting Bec Lido. Looking back, I don’t recall a particular day when the resolution to swim throughout the winter seemed finally to crystallise. It’s not easy to explain – even to yourself – why you would undertake such an unpleasant daily commitment. The pool is a hundred yards long and unheated. From October to April, the water is varying degrees of icy, sometimes frozen. The first immersion is bewilderingly painful. No amount of repetition makes it easy. Sometimes I would spend the whole morning before my swim haunted by a vague sense of dread. Worst of all were the days when the walk from the changing cubicles exposed shrinking flesh to wind and rain, or sleet.

Shuffling across the poolside in my flip-flops, I would pause by the water’s edge. For a long time, I was in the habit of easing myself in slowly and waiting at the shallow end for my legs to go numb before I submerged my whole body. It took me a while to realise that this was a method used by none of the small number of people who continued to swim through the winter months and who all regarded the use of a wetsuit as beneath their dignity. They would plunge in without hesitation, committing themselves to the scalding immersion that left them gasping for breath.

By January and February, most swimmers are doing a width or two at most, though some physiologically gifted people are capable of swimming eighteen lengths, a mile, when the temperature of the water is close to freezing. I discovered that my natural distance was two lengths, not for reasons of machismo, but because any fewer and the experience was abbreviated to something approaching pure pain.