E xercise can make you feel happy; any 11-year-old who’s had a quick biology lesson on endorphins can tell you that. The so-called “runner’s high” has the potential to pick us up and lift us out of a low mood – or even diminish pain – before our bodies stabilise and return to our baseline from around 30 minutes to a couple of hours after we cross the finish line. For a long time I’ve used exercise like pulling the eject cord on my brain : Anxious? Go on a stop-start 5k. Ruminating on destructive thoughts? Book a gym class. Unexplained edginess? Walk until your legs get sore – but the euphoria would always wear off, thumping me straight back down to where I started. Until I realised I’d been doing it all wrong.…