Yup, that's it.
Once a fortnight do your longer long run, building towards 20,(if you can go to 22 that'si deal). The other Sunday you do 10, building to 13.
On the short Sunday's throw in some hard paced running for a mile, two or even three, in half mile efforts, with half mile rests. Mind you if that seems too much, don't worry, you'll be OK, this is about giving you pace and a boost when/if you need it.
In the week at least two other sessions in the range 4 -7 miles, prefrably one speed work, builds your fitness and one hill session builds endurance.
If you run the day after the lsr, keep it short and not too hard, recovery is important.
Taper, that is reduce your mileage as you come up to your event. depending on your age, older = longer tapper. 3-5 weeks. So the longest run (20 - 22) takes place 4 weeks before, 3 weeksbefore 13, 2 weeks before 16/15 SLOW, 1 week before 10 SLOW, Then 4/5 days 3 miles. Then nothing.
Eat pasta the nightbefore. Bran flakes and banana on the day NO SUGAR.
Good luck, leave it till September and make sure it goes right, where do you live, if you are North/Midlands I can suggest an event, maybe.