As a general rule of thumb, to begin training for a marathon, it is advisable to first have a running base of around 25kms/wk.Considering that you are alreading running 25 k or thererabouts,you could choose to use any of the training plans for novices (halhigdon.com). Around 18-20 weeks are recommended from start to race day.
Considering that there are barely 16-17 weeks for the Nov23 race, I would play it conservatively and sign up only for the half marathon this year. That way, you can train much more optimally and keep injury free as well.
The initial rush of running can make one feel overconfident and cause overtraining and eventual burnout. I burnt out badly during the Mumbai marathon 2005 -especially due to overtraining.
Ofcourse, you could be blessed with better physical fitness than me,but I guess there will be plenty of races ahead in the future,so ensure you have happy memories of your first long distance event .So,in summary, I would say, only attempt the marathon after having checked out how the half marathon feels.