When Natalie Hollabaugh tested positive for Covid-19 in March 2020, her recovery felt extremely slow. Eighteen months later, she was still suffering from a litany of symptoms, including fatigue, shortness of breath, headaches and joint pain. She saw a cardiologist and a pulmonologist, who both ruled out other health problems, she said. And they advised her to start exercising, suggesting that some of her symptoms may have been a result of being out of shape. So Ms. Hollabaugh dutifully began using an exercise bike, speed walking on a treadmill and walking her dogs several miles a day