There are many things to concern yourself with when training for a marathon, but if I had to pick one single thing that is most critical to your performance, that would be your training. Training = putting in your mileage. Running a 26.2 mile marathon is not a natural thing to the human body. Our bodies weren't really hammering our joint over 26.2 miles of concrete for 3 to 6 hours or more. That ain't exactly the best thing to do to your body.
So the best thing you can do is gradually get out there and start training. Not too much, too soon. Start out slowly. Start out walking/jogging if you need to. You don't want to get injured right away.
In April 1993 I had never run a 5K race, let alone a marathon. I decided I wanted to do the 1994 Los Angeles Marathon. It was baby steps at first. I remember running one block and my knee or ankle would start hurting. So I stopped and walked the rest of the way. Tried it again the next day and lasted 2 blocks. Before I knew it I was able to run without stopping as my body adapted to the pounding.
As I train for the Santa Barbara International Marathon these coming 7 months, I will continue elaborating on the importance of putting mileage in the bank!
There are hundred of books on training for a marathon out there. Years ago this Hal Higdon book was the one that I enjoyed the most, found most easy and enjoyable to read and was most helpful to me.