Picked up our '01 FXDXT on August 3. Have rode 4500 miles since then, most of which is two-up. First investment was a Corbin dual touring seat which made a big difference for the passenger (my wife!).
The most I've done in one day is 375 miles & had no great discomfort.
Wife keeps looking at the rear seat on an Ultra, but that's a couple of years away!