Adding onto Lift's & Flat's advice, you could have 3 running days, one interval (INT), one continous high-intensity (CHI), and one long slow distance (LSD).
For the INT, do say, 4-6x400. Rest 2x longer than it took to run the lap.
-Goal: improve total time of session
For CHI, warm up, then run for 15 minutes at the highest intensity you can maintain for that duration.
-Goal: increase distance.
For LSD, pick a distance, say 4 miles, and run it at an easy, conversational pace.
-Goal: normally you add distance, but for your specific goals you could focus on improving time. Just don't make it into another sprint day.