REYKJAVIK, Iceland — An icy wind was cutting across Iceland’s capital, Reykjavik, just after 7 on a winter morning, still long before dawn. Puddles were frozen solid. Noses stung. It was, after all, minus 11 Celsius (about 12 Fahrenheit). That’s cold, even in Iceland. But there, under the open sky on the deck of the Vesturbaejarlaug public pool complex, some 20 people in bathing suits were doing jumping jacks in unison, their breath steaming as they counted and whooped, before dropping to the frozen ground for pushups. There were a few lap swimmers. But these intrepid friends crowded into a hot tub, laughing as their sags and smile lines and surgery scars sank below the steam. Other than me, there were no outsiders around — and certainly no tourists. “That’s why we show up so early in the morning,” joked Ragna Thorhallsdottir, one swimmer, sipping a coffee after she had changed back into dry clothes.…