Congrats on making the decision to go primal.
If your goal is to just finish the 6.5 miles you could most likely get away with less running. If it were me coming from zero running shape, I would start slow. For the first four weeks trying running /walking for the thrity minutes 3 to four days a week. Let your body be your guide. For the next four weeks, incorporate some intervals like 6 X 100m with 30sec rest one day a week. The last four weeks would be one interval day, one day running for 30 minutes, and one long run (4-6miles).
I hope this helps. Be careful and strat slow, as overuse injuries affect 90% of runners, and derail your efforts.