From a Proofs professor of mine who helped me train for my half during summer--start at a brisk 5k. Up your mileage a kilometer per week while maintaining a desired race pace. That puts you at 25k (or 15.5 miles) by the week before at or above your race pace.

That is a once or twice per week run. Always have an LSD run, and if you're really concerned about speed, do a day of intervals each week as well. Total of four days per week, so just alternate days for sufficient rest.

Worked for me, YMMV.