Training for marathon

Yesterday I have registered for my first marathon. It is going to be on 17 November. I am very excited about it. Bought me a Garmin Forerunner 210 this Monday and on Tuesday I have registered for a marathon. I want to get best of  my new watch. When I don't work out for a long time I feel guilty and with the first work out I push myself hard. I hope the watch will prevent it.