Here is atraining plan which yielded surprising good results for me. I did this for 10-12 weeks last fall.
I did two workouts a week and sometimes a longer run (90+ minutes) on the weekend. All the easy days were 45-90 minutes of running. I would switch between the Friday A/B every week.
Tuesdays: 6x60m max speed sprints 4 minute recovery. Run these literally all out. Make sure to warm up well. Injury risk is somewhat high.
Friday A: Tempo of 3-4 miles, then some (5-8) hill sprints. Then a mile of work at 5k pace. Could be 1x1600, 2x800, 4x400. What ever I felt like.
Friday B: Goal 5k pace running. Started the first week running 20x300m. Increased length of the repeats each week. The second time around I did 400s and some 200s. Always did about 6000m-7000m of work. Worked up to a workout of 8x800s. Did a tune up of 3x1000, 4x400, 4x200 on the last week before the race.
Not saying this was a great plan, but this got me close to my PR where I hadn't been in a couple years.