Hi,
I would say that a year is ample time to prepare if you are able to stick with it.
If you do sign up for the GNR, they will provide you with week by week, training plan and schedules to keep you on track.
In the meantime, I would maybe speak to your GP for a health check first to see if you are ok to go ahead with starting training, especially if you are generally unfit and a bit overweight. They will also be able to give you some advice about a healthy diet to sustain you while training. This is very important.
You could try doing a little research on a website called Runners World (just type it into Google), and that will give you some sound advice, even for beginners.
Good Luck and Enjoy!