Giving yourself a year to train for a 1/2m should be just about right if you are running from scratch.
Would advise taking the first month very easy...not too far and not too fast so as to allow your body to adjust. At that stage try to find a local running, not athletic club, to join to give you company, encouragement and advice. You'll then find yourself running longer and faster and best of all entering 5k, 10k and even local cross country races which will all help you get stronger and fitter for the 1/2m next September.
All the best!