The B736 is the baby of the family, so not really designed for the long hops that the B738 and B739 are - they're more intended for feeder routes to put people on bigger aircraft to do the long hauls. That said, it should still be capable of doing four figure distance trips, so something isn't quite right there.
I know of airlines who use B736s for three and four hour flights, so it should still go a lot further than that one does in time, as well as distance. As fliger747 says, you don't want to start pushing the speed much above M0.74, and you want to be up in the FL3xx levels to be getting decent ranges, but even at 10,000' it shouldn't run out of fuel in two hours. Anyone know what the ETOPS rating of a B736 is?
Ian P.