“It’s going to be incredible,” I overheard a local explaining. “They’re going to swim all the way to Tresco and back again.” Actually, it’s worse than that, I thought to myself, as I tried to walk off my pre-race nerves. We’d be alternating between swimming and running across six of the Isles of Scilly, off the southern tip of Cornwall. No stopping. No change of clothes. Just a wetsuit and a pair of trail shoes. Welcome to the sport of swimrun, invented by two Swedish adventurers with a penchant for drunken bets.