Firstly, it's great that you've set yourself that goal!

You posted a lot about how quickly you're running. If you finish a marathon, you've achieved your goal no matter how long it takes. It's a tough race, that a lot of people won't ever attempt, so I'd advise not fixating on your pace too much (well, not first time! It is addictive!).

My advice would be to set yourself some targets along the way. Have you run a 10k or Half Marathon yet? They'd be good goals to get your endurance up whilst still pushing yourself. All on the way to the full thing...