If you run 54 in the 400 but slower than 2:05 in the 800, you need reps mile pace and tempo runs. Your speed is enough to run 1:58-2:00 in the 800, so you need more endurance.
3mi tempo @ 6:10 once a week
10x400 @ 70 with 70s rest
5x600 @ 1:45 with 2:00 rest
10x800 @ 2:45 with 1:30 rest
10mi long run under 1:15:00 once a week
Your times will drop at distance events. Case in point: I can only run 59.x in the 400, but I can run 1500 (converted) in 4:22. If you study the relationship between your times for different distances, you'll find that your pace per mile for various events increases linearly as the distance doubles. How much it increases depends on your distance training.