The fast start may have been out-strategizing yourself, but I think that confusion over the course is a problem with the course not a stratagem. For a turn-around, I would expect a very clear marking including something in the road to physically go around. I think that a course monitor would be appropriate at that point as well. If the turnaround marking is less than that, then they need to cover it specifically in a pre-race briefing or starting instructions.
Did the other guy call out to you? If he realized that you were confused or off course, I think that the right, honorable, and/or sportsmanlike thing to do would be to call out to help someone get on course. I would be willing to lose a bit of time to help another runner get on course — perhaps even chase them off course a short way to try to get their attention. It's a road race, not orienteering. I would hope that someone might do the same for me if I got off track or confused.