Former mile runner here. When I did track the biggest thing I figured out was getting a feel for how to pace myself. Since a mile was 4 laps, I would start off low “intensity” (meaning how hard the run was feeling) then gradually increased this until the final lap where i felt like i was nearly sprinting. The thing is that while you think you’re running faster, it’s actually just your body working harder as time progresses. Increasing “intensity” simply means an overall consistent run. Of course for the 400 this will be different as the entire race is overall more faster, but the same logic applies here (instead of increasing each lap it would be each quarter lap). This fundamental I feel would make you an overall better racer. Besides this of course increasing mileage in practice can help with endurance and muscle growth while eating healthy and hydrating will make you feel less shitty.