Chaffing is a thing. I wear yoga type pants or shorts to avoid it. Also, body-glide might be your friend.
Otherwise, what the others said, "seed" yourself. A lot of half-marathons have pace groups, you can look for someone holding a sign with what you expect your pace to be (i.e. I'm usually in the 9:45 or 10:min group).
If it's a really big race, you might be stuck walking the first half mile or so until things spread out a bit. My first half marathon was the Disneyland half, and even though we were seeded by time they put the walkers in front of the runners so it was a big mess and very crowded.
Personally, I like to have one headphone in for a longer run so that I can hear my zombies run app and then the other ear bare so I can hear what's going on. If it's going to start cold and warm up, you might want to wear a sweater that you won't be sad if you throw it away mid-race.
Good luck and listen to your body!