Right out of the box, primal is great preparation to do a semi-serious/casual marathon. Focus on becoming a fat burning machine and getting the standard primal exercise plus some endurance training.
I did a half marathon after training (at age 67) with a group that takes a fairly primal approach (to training, not food). Two sprint sessions/week, once a week increased distance slower than you think you should go, short walks during the week, some cross training and rest days. That will allow you to do respectably well without doing the damage to yourself that marathoning can do.