I think the most useful and easily obtainable workout is doing repeat 2:20s on non specified interval training dates.
It's easy and manageable opposed to trying to do mile repeats and busting on the second mile. The 2:20s are nice and easy.
Now, as to your interval workouts, everything boils down to 200m speed. You need to be able to run sub 30s consistently in workout shoes. You wont be able to dabble in repeat 400s to the beat of your ability if you cant run a 200. With the 200s, you almost dont need to concentrate on rest period. You just have to do them fast and efficiently. Your arms need to be pumping, form in check, and legs properly lifting and extending.
For your level, a 20-minute warm up, 40-minute fartlek on grass, 12x200 on the track and the weight room is a perfect workout.