Are you certain the rear pads are the same thickness as the fronts when new?
Also, if it's not acting up, and if there's no discoloration on the rotors (purple / brown) that would indicate overheating from drag, then I'd say don't sweat it. Drive it and enjoy it - The JX is a great car!
With that few miles, and a car that's almost 7 years old, my concern would be that the calipers STILL require servicing - lubrication of the shims, sliders, etc. Pad wear is certainly an indicator of remaining brake life, but it's not the determining factor.