I found my 8 week marathon training program that might work for you.
it goes up to 10 miles, but you really only need to train up to 9. If you can run 9 you can run 13.
you only have to run 4 days a week, varying distances. the idea is that you run a base distance, then a little longer, then shorter, then the 4th day you run a long run, with 2 recovery days after that. you can tailor it however you want to, just make sure it is incrementally longer “long” runs every week, with longer “short” runs as your base.
the book I have that I based it on is called The Non-Runner