@Work Advice: Swimming against a workaholic tide

Reader: I started a new job recently, and noticed right away that everyone in this five-person office, including the owner (my supervisor), works at least nine hours a day. No one has specifically said I have to put in 10-hour days, but I feel like I'm committing a major faux pas when I leave at 5 and everyone else is staying until 6:30. I have a longer commute than the others, so my regular eight-hour day is already 11 hours long.