23.6 x 2 = 47.2…..55.5 – 47.2 = 8.3 secs
Getting down to a 53. or <53 should not be a problem. You are in the exact same boat I was in….speed enough to run a decent 400m, but lacking proper endurance. What kind of off-season work have you done in the past, or have you done any? Are you lifting weights? Is your diet balanced? Are you drinking enough water (8 cups is not nearly enough)? How is your form? All the little things can help you take off time.
As I was once told, "Drop seconds from fitness." In other words, with the endurance deficit like that, as I am in the same boat (23.91 / 54.24), your best bet would be to just up overall work capacity and general fitness and let your times drop off that rather than focus too much on speed and speed endurance, yet not have the body to handle it.