I've run a few half marathons. Got another few coming up I've the next month. Do 10ks fairly frequently too.
I've got a bum bag that I keep my BG meter and a couple of packs of dextrose. Not the height of fashion, but it's critically important on long distance runs that you have your meter and some fast acting carbs on you. Please do not run without them!
Most half marathons start in the morning, so you want to make sure you don't have any fast acting insulin in you otherwise you'll end up hypoing, so either have breakfast and inject 5 hours before, or don't have breakfast and munch through some dextrose tablets on your way round.
I also test fairly frequently throughout my run and top up with dextrose as required. You'll find that for the first 7/8 miles you won't need glucose. After that, you're glycogen stores will run out and you'll need to have some fast acting carbs for the rest of your run.