When I was 29 I literally did exactly this. I started running in January, 3 miles, 3 times a week. I read the Bob Glover Book of Distance Running, and used the 26 week Beginner Running training plan. That November, I ran the New York City Marathon, my first, and it took me almost 5 hours of continuous but really slow running. It was a complete blast, one of the best days of my life. But, I was skinny and 29.
In your case, I would defer the acceptance for one year, and train for 18 months. Do a bunch of 5ks, 10ks, 10 milers and half marathons. It's an awesome thing to do. But do it with respect for the undertaking, and give yourself time to build up to it.
I didn't, but my husband and I were running as part of our honeymoon. So we went for it.