Hill reps. Start with 6x8 seconds. Put a mark at your finish point. Walk back slowly. Wait at least 90 seconds before the next rep. Once you can no longer hit the mark, the workout is done. Build up to 8 reps.
Flying 40s. Carry a stopwatch. Work on improving your fastest rep as the weeks go by. Take up to 5 minutes between reps. Once you slow down... say a half second... the workout is done.
During your normal runs after a warm up of at least 10 minutes, do the following drill. Slow down. Dramatically increase your cadence by keeping your arm swing compact and moving your arms as fast as you can. Do this for 20 seconds. It will be VERY tiring. Jog/walk a couple of minutes to recover and repeat. You can download a metronome app to measure your cadence. I'd suggest that you try to start with a beep every other foot strike at 120 beeps/minute. That's a cadence of 240. It's unlikely that you can even reach that turnover, but try it. Then, try 115, 110, or 105. A 105 beep every other footstrike is a 210 cadence. You should be able to do that, but holding it for 20 seconds might be hard. Wherever you start, do 4 reps of 20 seconds with a couple of minutes rest in between. Once you can do 4 reps, increase to 6. Once you can do 6 reps, increase the time to 30 seconds and try to increase the cadence another 10 BPM.
Eventually, you will hit an upper limit of cadence that you just cannot hold for more than a few seconds. For me, it's 260 so I do the cadence drill a little slower... at 240. Remember, it's a drill. It's not running. It's not distance or pace. Distance and pace are irrelevant. It's just shuffling along with as fast a cadence as you can muster.
The drill trains your CNS to fire faster. The hills give you range of motion. The flying 40s combine both with push off. Eventually, you'll put it all together when you race the 400.
Of course, these are only drills. They aren't 400m workouts. That's another subject.