The mantra “never turn down an opportunity” had been ingrained into Walter Holohan from an early age. His dedication to it was perhaps never put under as much pressure as it was in September 2021, when, two months before the birth of his son, he was asked to leave his job and come on board as the first employee of a scrappy start-up that promised to revolutionise running. “I could easily have taken my foot off the gas, but I didn’t, and here we are four years later,” he tells The Currency. That start-up was a company called Runna, initially…