This is not as unusual as you might think. Many runners do their first, and often only, marathon using Team In Training (TNT), the Leukemia and Lymphoma Society's program. Lots of these runners have little or no racing experience and often little or no running background. What matters is your base training, your approach to marathon training and your approach to the race itself. Hopefully you have had time to develop a training base of at least 15 miles per week (preferably more like 25) before starting your marathon training. Then you need to pick a good marathon training plan suited to your current running abilities, and following it as closely as possible. The race itself also has to be done at a level suited to your running abilities. TNT for instance, is often done run/walk. The fact that you mention overtraining issues is a bad sign. Pushing too hard is as likely to leave you injured as anything. If you do it in the marathon, you stand a chance of having a miserable several hours, particularly the last 6 miles or so. Be conservative in your approach to training and to the race and it will be much more enjoyable.
Good luck.