My favorite track workout is essentially a half-hour of 400s: 10-minute warm-up jog to the track, then run 400 repeats at 5–10K race pace with 200 easy jog between (continuous). Do as many as you can in a half-hour, then 5 minutes easy jog home.
Fartlek: Silly name, serious workout. 10-minute jog warm-up into 1 minute hard, 1 easy, 2 minutes hard, 1 easy, 3 minutes hard, 2 easy, 3 hard, 2 easy, 2 hard, 1 easy, 1 hard, 1 easy. 5–10-minute cool-down. Paces are as fast as you can consistently hold for each interval.
Tempo: 10-minute jog warm-up, straight into 20-minute tempo: 4x(3 minutes steady state/2 minutes 5K pace). 5-minute cool-down. This workout is deceptively hard due to the incomplete recovery.