While I think you are nuts for doing this set, though I do wish you the best of luck, I would defintily have some goals in mind for the set.
You might consider doing speed play swims within each 1500. 1) A gradual descend of 100's by time. 2) Descend by 300's. 3) Descend by stroke count. 4) On the odd number 50's, add a brisk swim. On the evens, calm swim on the way back. 5) etc, etc, etc until you have finished.
If your only game plan is to swim 15x1500 meters, it will be a long day. The more tired you get, the longer the day becomes.