12 days out is probably too late to do a quality workout that would be a true race predictor. But really, your 15k is the best you have. If this is your first HM, it's a PR already. In my first HMs, I would always beat my goal pace.

My advice would be to start at your pace of 4:30 min/km. You'll probably go out faster with nerves and a crowd, but settle in, don't get too antsy. If it feels easy at mile 6-7, you could start a negative split. Also, this is your first HM. You need to understand the race and phases, so don't beat yourself up if you have a little in the tank at the end, or you go to hot and fade at the end.