I've been training for the marathon goal for a bit. Talking to runners, I understand a lot of factors are important with respect to race day itself, just like any competition. There are the mental effects to focus on with running in front of spectators and being in competition itself, and then moderating your pace and not starting to subconsciously draft behind other runners. Also diet on race day and other things are all considerations. I think getting in gradually longer races and starting small is invaluable.
I'm doing a half marathon in October and will run a full sometime thereafter based on my experience at that event, but right now I'm leaving the timing and planning for further progress until after an evaluation of my performance and progress. This is more or less the advice I've gotten from other runners.
"Use what talents you possess: the woods would be very silent if no birds sang there except those that sang best." —Henry Van Dyke