Follow Cali's advice - start off longer and shorter and move your way into the middle.
For longer - if you aren't running longer then 800, then use cruise intervals. Say 6-8 x 1km at about 5-10km pace with short rests of 60-90". That gives you a good aerobic workout and can replace long slow running - which you don't need to do any of. Alternatively 4-5 x mile at 8-10km pace on 2 mins' rest.
About 3-4 months out from the big comp, move towards 200 training at the shorter end (flat out 120s one week, 150s the next, up to 200s) and mile training at the longer end. The mile training once a week can be vVo2max training, which is 25-30 mins of 1-3 minutes intervals at 2km pace and the same rest either walking or at 60% of 2km speed. For example 1 min hard (333m) and 180m jog recovery in 1 min, times 12-15.
Then at least 2 months before the main comp, do specialist 400-800 work. I'd suggest 4 workouts a week, one being 200-400 training, an 800 session, a mile session, and the cruise intervals.
Oh - and make sure you do core workouts from the start. You need to get your hammies, glutes and quads strong enough to sprint,