WATERVILLE — Jonathan Alexis often gets his workouts in running up and down the stairs at MaineGeneral Medical Center, where he is an overnight security office supervisor. The 48-year-old, who is also a martial arts teacher at his family’s jujitsu and karate school in Oakland, says it’s been good training for his upcoming appearance on “American Ninja Warrior,” a reality show on NBC that follows competitors along a series of obstacle courses around the country.