Have course marshals around the course to direct people, report injuries, and take charge should the need arise. You can't run a race without them. Make sure they know what they are expected to do and how. Give them free t-shirts and food. Unless you reward good worker, you won't get them back again.
People like to be appreciated.
You would not be able to do the race without them. Make sure they know they are wanted and needed. You should learn from your mistakes. Throw a post-race dinner a few days after the race to get everyone together to talk about what went well and what didn't. Take notes and use them for planning your next event.