Okay. You've got the basic speed that you need, and your problem is to extend it--speed endurance, in other words.
**Somebody (the Soviets, I think) did a study years ago that found speed endurance was best developed by doing repetitions that had several changes of speed. Examples:
1) Run 200 or 150 or even 100 as follows: maximum acceleration for the first 20% of the distance, "float" for the second 20%, sprint for the next 20%, float for the 20% after that, then sprint *through* (not to) the finish.
[Analogy: when you sprint, it's like flooring the accelerator in a car; when you slow to a jog, it's like hitting the brakes; when you float, you take your foot off the gas but keep it off the brake, as well--maintaining speed, but with no effort.]
The benefit here occurs *within* the repetitions, so take good, if not complete, rests after each rep. Build to 800-1200m (total) in a workout.
2) 100m accelerations: from a jog or walk, build gradually until you touch top speed (this may be at the 50-60m mark); stay there only as long as you can maintain complete relaxation (probably between four and twelve steps); then ease it back down. Once you get the hang of these, you can start to string accerations together and eventually run a 400m lap with four accelerations in it.
**You also need to develop the ability to continue sprinting in the presence of lactic acid. Do sets of this: 4 x 150m fast, 50m jog/walk--60 seconds from the start of one rep to the start of the next.
Here the benefit comes from the *combination* of runs-and-rest, so the runs only need to be approximately at your goal pace (between 19 and 25 seconds is fine), but be very careful to start the next rep exactly 60 seconds after the previous.
There are two ways you can progress on these: start with one set, and work up to two (or *maybe* three)--nearly-full rests between sets; or, stay with one set and gradually work down from 60 to 55 to 50 to 45secs for each run-and-rest within the set. (I recommend the latter approach within the last couple weeks before your test.)
**Technique: If your legs feel tired, it really helps to switch your brain to your arms. 1) When your arm moves backward, it pushes your body forward. 2) It's very important to keep the elbow relaxed (not locked at one angle): a relaxed elbow opens a bit as the arm swings back, and closes some as the arm swings forward. 3) So you should focus mentally on the backward swing of relaxed elbows: "relax-back/relax-back/relax back."
Good luck. Let us know how it turns out.