If I were you, I would approach in photo journalistic manner and I would keep my distance. First of all, you can't be anywhere near where the main photographer is. It is just annoying when a photo is ruined by some guy sticking his head into the frame. Second of all, While the pro is doing the formals, shooting the bride with the groom and etc, walk around the place where the wedding take place and try to catch candid moments of all the guests.
Your photos will be different and unique cuz the pro often doesn't have enough time for those shots. Also, usually guests at weddings love to get photographed and bother the main photographer (which in some occasions can be annoying). Use that to you advantage.
You gear is nice and capable, no worries about that.
Good luck, post some pics
Oh, and use Av + exp. comp. + remember how camera meters.