Why do it, is a great question.
If you're answser is, because I want to and you really don't have any burning desire to "be" a runner(I was for 20 plus years), why spend months training, losing some muscle mass, beating up your body, NOT improve overall fitness, and the list is endless.
I like to run some short stuff, say 400-800 and maybe a mile, along with Tabata work, I get all the high intensity cardio(dislike that word) that I need and still maintain a pretty good level of fitness.
Just had a friend train 4 months for a marathon, she had muscle issues, flu, colds, spent 1000 bucks on trip and when I asked her why, all she said was that she always wanted to run one. Good answer. I asked her if she was going to run another one, she just smiled.
BTW, she can't do one pull up.
So if you want to bang out a 13 mile race, go for it, no need to train more then 90 days, and really three days a week is plenty. Tempo run/ Intervals or Hills/LSD on weekend, more then enough for that distance. Heck, I had a friend train for NY on 3 day a week.