A.D. Miles, head writer for The Tonight Show Starring Jimmy Fallon, who has been with the host since the launch of his late-night show, is stepping down after eight years of working with Fallon. Miles will be moving back to Los Angeles, along with his wife and two daughters, to pursue other opportunities as an actor and writer for film and the half-hour format, which also includes an animated movie with Jimmy Fallon. “Riding Jimmy Fallon's coattails for the past eight…