Fujisan Marathon 2013 will be held on Sunday 24rd November 2013. The organizers still accept applications, until 25th September. The marathon course is really beautiful, if you are deciding whether to join or not, definitely do participate!
Oficial page of the race
I have participated in the 1st annual Mount Fuji Marathon last year. You can read more about my experience in my previous article. Right now, it's almost exactly 3 months until the race, which I consider a bare minimum to train for a marathon from zero.
If you are taking train to Kawaguchiko, check my 7 tips for train travel in Japan.