Work on your anaerobic endurance.
Try this for a WO.
500,300,150,300,500 with about 5 mins between each run. You should be aiming to run them all at the same pace. For example if you go through 300m in 40s on the 500, then you’ve gotta do that on the 300 and the 2nd 500, and the 150 needs to be run in 20. I’ve used this one sometimes, but I prefer doing it on hills with more reps, because that’s more intense (with decreased distances of course). But this is supposed to be a great session for prep on the 400.
Another good session is 6-8×250, where you gradually accelerate until you’re almost at top speed at the last 50m. The recovery between each run is about 7 mins.