I saw him during the final stages of the half-marathon. A man who’d descended into an even more pitiful state than me. His pace had slowed to a forlorn shuffle and his face was contorted with pain.

Yet this poor guy’s struggle had nothing to do with him being ill-prepared or under-trained. The lactic-acid nightmare he was slogging through with gritted teeth was entirely down to his own volition. He’d decided to run the half-marathon wearing a 20-kilogram weighted vest.