You will start your training probably around 18 weeks from the race.  I run a 1/2 and several 10Ks during my cycle.  I don't always race them but run them harder than my long runs.  Most plans will allow you to race about every 4 weeks during your cycle.  With the shorter races just make sure you get the miles that you are suppose to run, say if you plan calls for 18 and you run a 10K you still need to get the additional miles in, that can be tuff if you raced the 10K.  I find 1/2s very good for training runs for me during the cycle.  This past year for my A race which was only a 1/2, I did three 1/2s before the race.  Good luck

When I was training for my first marathon in Oct. 2009, I remember reading on one of these boards that racing a half is roughly equivalent to doing a 17 mile training run. That would determine where you'd want to fit it in your plan.