Americans are flocking to gyms and fitness classes. It wasn't always that way. This episode was produced by Hady Mawajdeh, edited by Miranda Kennedy with help from Naureen Khan, fact-checked by Melissa Hirsch, engineered by Andrea Kristinsdottir and hosted by Jonquilyn Hill. Photo of an outdoor group fitness class by ANDREW CABALLERO-REYNOLDS/AFP via Getty Images. If you have a question, give us…