Do a workout twice each week. This could be a tempo, mile repeats, 1000 reps, 800 reps, or 400 reps, or a combination.
Tempo: Basically a little easier than your 5K race pace. Go for 3-4 miles.
Mile reps: Try to match your 5K race pace. Do 4. Take 2-4 mins for rest between them.
1000s, 800s, 400s: Go faster than 5K race pace. Do 4-6. Take 2-3 mins for rest in between reps.
As long as you feel OK, fill in the rest of the days with easy running. Like a pace you could carry for 12-15 miles if you had to do so. Run for 4-8 miles each time you do this.
Sometimes you can include a long run. Do this at the same pace described above(easy). Easy would be about 90 secs slower than 5k race pace.
For example:
Monday: 5 miles easy
Tuesday: 3 miles at 6:15 pace (a bit slower than 19min 5k)
Wednesday: 6 miles easy
Thursday: 6x800m @ avg of 2:50 (a bit faster than 19min 5k pace)
Friday: Rest/Walk/Lift
Saturday: 6 miles easy
Sunday: 13 miles easy(this long run might be bi-weekly)
Mix up the workouts and try to do hit the last rep hard if you have something left in the tank.
Be patient but not lazy.