Should i work on my sprinting speed or endurance to improve from 57 to 52 seconds on the 400m?

I need to improve 5 seconds. I think i'm a pretty fast sprinter maybe low 12s, 12-12.5 last year was about high 12s low 13s.I did conditioning with cross country even though i sucked at long distance.. fastest 2 mile at a meet was 17:44 and fastest 3 mile was 25:30 (pretty bad). I ran a bit over winter to keep my endurance.Right now how likely is it to improve 5 seconds and should i focus on practicing to sprint faster or work on my endurance more? and how do i do it? Thanks