People worry about this all the time, but the answer is really quite simple.  Firstly, you should not try eating anything new or different from what you normally eat for breakfast.  If you are asking this question, chances are that you have done a fair amount of training.  Ask yourself what you would eat before a really difficult morning workout and do more or less the same.  You do have to factor in how long you figure the race should take you to complete, what opportunities you will have to fuel during the event and how long it will take to go from where you are staying to the race venue.  You should aim to finish your meal about two hours before you start.  It is also a good idea to have a bottle of electrolyte beverage with you to sip from all morning to ensure you will be adequately hydrated for the beginning of the race.  Just in case, bring an energy bar or gel to consume closer to the start of the race.