It is hard to say without knowing your body type and what types of training you respond to the best. I usually do a 45 sec. time trial test on the track to see the potential of my young (middle school) sprinters. If they can reach 300m by 45 sec., I feel as though they have the potential to do pretty well. You may start with something like that to see where you are at.
You seem to have a descent base for someone making the transition to 400m, you simply lack the overall speed endurance to excel at the event. If your goal pace is 60sec., it may help to do some 600s and 500s crossing the 400 in around 70 and really working the remaining 100 or 200...staying comfortable(early training 2 or 3 months). You will want to make sure and have a descent amount of recovery between reps and also mix in some 200's and 300's that are at a fast but comfortable pace.
Example of early: 2X600 @ 70 cross, 2X300 @ 47-50, 2X200 32-34 (plenty of rest in between
Example of speed development and LA tolerance for 400m: 9X90m with 90 sec rest (these should be run fast but comfortable)
You will want to continue the over distance training for a while but maybe cut the reps down and the distance down to 450m and speed the pace up.
Bottom line is that you have to listen to your body and work different aspects and phases of the 400m, relaxing, running fast, and developing speed endurance. These are just some ideas, by no means the gospel...good luck!