The little indents usually start to show pretty quick,certainly should have them with miles you have on it. Sounds like you havent been sitting on the back part where sit bones are supposed to sit,either that or your saddle is too narrow,with sit bones off the sides.(that would be painful, ask me how i know). Only other thing i can think of is that your bars are way lower than saddle,so that your not putting weight on back wide part, cause you should definately have indents.