Workout:

 

12 x 400m
4 @ Threshold pace;  1 min. recovery
4 @ 5k pace; 2 min. recovery
4 @ Interval pace (about 4 sec. faster than 5k pace per 400m); 2:30/3 min. recovery

Another option can be:

6 x 400 @ 5k pace
6 @ Interval pace
same recovery as above