training both speed and endurance will really help you out in the 400.
if you can do 24s 200m from a standing start & without having done much speedwork yet, that's awesome. says you can probably do 22.5-23.5 200m with a running start when in peak shape, but 159 is weak for a guy with that type of speed, and 448 is even worse.
i found that when i trained for the 800/1600, my 400 time got waaaay better. i could only manage 25.8 for 200m with a running start that season in training, yet my 800/1600 are were better than yours (1:57/4:19). was running 55s at the end of tough workouts that season, which was my previous PR in the open 400!
doing the fast sprints to keep the speed + adding in tempos, controlled intervals, and even long runs as the season progresses is a good way to build endurance. just structure your training so you are fresh for the speedwork. and keep the tempos/intervals/long runs controlled so they don't run you down.
and don't listen to people who say "distance training kills speed". incorrect. not training speed kills speed. get fitter + get more endurance (while maintaining the speed) and you should crack 50 for 400m.