I have never driven further than 100 km at a time, but I have been a passenger for many long journeys, with my father driving the car.
The longest non-stop journey as a passenger was from Islamabad to Karachi in 22 hours. No breaks except fuel stops. I stayed awake the whole time to compare my stamina with my dad's :). My brother, sister, and mom couldn't stay awake.
Aside from that, I have covered at least 10's (if not 100's) of thousands of km's in the back seat of a Cultus. I am immune to all sorts of uncomfortable cars and hard suspensions now (H)