The enduring frustration of running? Its unpredictability. Some days you feel good and run bad; some days you feel bad and run good; some days you feel bad and run bad. And – on magical, rare days – you feel good and run good.

I had a felt-good-ran-bad day yesterday. It was a parkrun, only a parkrun. On Friday night I thought it was about time I broke 17 minutes for a 5k. I’ve gone 31 years without doing so, after all. I made the mistake of telling people this was my intention. There was realism in my optimism, having run 17.14 on iffy training and not a stride of warm-up a month earlier.

I didn’t feel good on the run to Dulwich Park. Tired. I’ll be fine when the race starts, I thought. I’d had a month of quality training, the confidence boost of a race win, and the recent experience of short, fast racing in my legs. What could go wrong? I wasn’t fine when the race started, though. Tired. I forced myself to run the pace required to match 17.14 and knew I’d struggle to maintain it. On the second lap of Dulwich Park, I slowed to 10k pace. The very, very modest incline felt like a proper hill.

Slowing further, I realised I’d find it a challenge to break 18 minutes, let alone 17. Feeling marginally better on the third lap, I roused myself in a sprint finish, crossing the line in 17.49, wondering where it had all gone wrong. How can it be that I run 17.14 on a whim, only to return a month later, fitter, more prepared, warmed-up, well-oiled and psyched, to run 35 seconds slower? That is the enduring frustration of running.