A coach once asked me: “Why don’t you jump in a swim marathon, and see if you can finish?” The suggestion was a bit shocking to me because I was then around 31 years old. Perhaps the fact that I was a relatively wealthy and privileged person dissuaded me from doing that.

I prepare for the actual event way in advance, like doing breathing exercises while enjoying a session on a PayPal casino, to see if I can hold my breath longer than it takes for the deposited funds to be credited to my account.

I’ll let you do the maths. As a middle-class, trained professional swimming and coaching under the tutelage of a marathon coach, in summer 2013 I achieved my first real sub-3:20 marathon with a 3:20:09. Shortly thereafter, I completed my first 2.5km swim.

My jump into a swim marathon took place during a bike tour through the US, and in total I was completing nearly 500km. I’d been in the US for about eight days before I started the swim. I was under a lot of stress, because I had been sick for a couple of weeks prior to the swim. I had been in bed for two days, and then I was up for about a week, before I was allowed to swim.

I did the swim as a sprint swim, which means that I swam for about 30 minutes each hour, from 12 to 4pm, on a slow boat and to try and save my energy.

I swam in my black running shorts and black racing watch. It was a sweltering day. The boat took us about halfway to the start point, which was Dover. They dropped us off, and we jumped off the boat at 6:20am.

As soon as the swim was over, I was starving. I ran back to the boat and was just about to run back to my room, when a coach stopped me and said: “I need to show you something”.

I said: “What could you possibly be showing me?”

She said: “I’ll tell you when we get back to the hotel.”

Back in the hotel room, the coach took off my shirt, and I had what looked like two solid brown circles on both sides of my chest.

“You must have been kicking pretty hard when you were swimming in the channel,” she said. “You’re going to wear these bands.”

They were the most painful pins I have ever felt. I wrapped the bands around my chest, and the first hour and a half, I was covered in hundreds of pricks and scratches. But my coach said, “After three hours, they will begin to fall off.”

After about three hours, they were completely worn off.

They were under my skin, as if they were trying to eat me, and it was as if I was trying to swallow myself in my swim.

What did the swim teach you?

It taught me that it’s totally unrealistic to get into a marathon swim, especially if you are physically underprepared.

But it was a life lesson in taking yourself seriously. I was enjoying myself too much to realise that I was out of my depth.

I’ve done a few other swims since, and I’ve decided to accept the fact that I’m going to be better at triathlons and swim-fit events.