400m pace training

If I have a goal time of 5:59.99 for the 400m free, but don't have any experience racing this distance, what would be a good test set to gauge where I am now, and what approach to pace training would be good for this event? I was thinking 4x100 at 1:30 pace with X seconds rest as a test set, but I don't know what value of X to use to give a gauge of ability to do it continuous in a meet. Secondly, for pacing, should I train to hold a steady 1:30 pace or figure that I'll do the first 50 or 100 faster and then hold a slightly slower pace the rest of the way with perhaps a slight speed up in the final 50 or 100? Would sets of Y times 100m on 1:30 with Xs rest where I slowly decrease X be a reasonable approach to training for this? Would 6 be a reasonable value for Y? I've always swam much faster in meets than I can manage in practice, but up to now I've considered the 200 to be a distance event! I'm more used to pushing speed than endurance and pacing. I'm hoping that the extra endurance from training for the 400 free might eventually lead to a better 200 fly. Thanks in advance for any advice, and for repressing the urge to publicly laugh at how slow my goal time is!