Experts say not to resume exercise too soon. But what’s ‘too soon’? — For Adam Harmon, Covid-19 began with a bad headache. Then came the fever and muscle pains. “I felt really achy for a day or two,” says the 31-year-old Park City, Utah–based endurance athlete. Harmon has been a triathlete, “semicompetitive” bike racer, and runner for years, but in the months before…