sw07, how long have you been running and what is a typical week you're currently doing?
Getting under 20:00 will require a good amount of interval training as the 5K is a very anaerobic event.
You still might need higher mileage as your performance above the 5K drops off.
This calculator predicts a 36:37/5 mile and a 45:55/10K based on your 5K.