In Initiative, a group of young people in the early 2000s finds themselves via the role-playing game, the latest example of its undying popularity It sounds like a big ask, the idea of presenting an audience with a five-hour play. (Or even a four-and-a-half-hour play with several intermissions.) Yet Initiative, a new off-Broadway coming-of-age epic of sorts, flies right by, as emotionally immersive as the Dungeons &a…