Please take this as a pretty everyman response -- I have no real history as a race organizer! -- my nonprofit orchestrated a 5K some years ago in the Maryland suburbs of DC though.  I would say, recruit a seasoned race organizer and/or running club to help guide your work on organizing your event.  This way you don't need to reinvent the wheel to find timing help, or bibs, or to devise prize levels, to map the route, certify it, or even to get a leg-up on publicizing it.  This worked very well for us -- at the time I wasn't a runner, and neither were many of our board members, so if your group has more experienced competitors in your midst, then maybe you don't need to do this.

Oh, and of course, be sure to talk to people involved in the FIRST of your events -- find out what they think worked well, what they ran into that they did not expect, and so on!  Keep good records of your own planning so that you can hand them on to the next planning team too!

Have fun!  I will see if I can get in shape to participate in the event, myself!

Linda