By LISA ELAINE HELD, MAY 3, 2015 In an average fitness class, running shoes and ponytails are standard. To work out in the Vixen Army, recruits wear wedge sneakers (they’re recommended in the What To Wear section on the website along with a favorite lipstick) and let their hair down. Members of the New York City regiment are packed into an old dance studio near Penn Station, practically on top of each other, to shake their booties with their Commander in Chief, Janet Jones, 3