If your temperature gauge takes a long time to get up to midrange and/or never does, good place to start with is the thermostat. We are assuming your temp gauge is operating correctly, which is probably the case because the inside temp of the car agrees with the low reading of the gauge. There also may be more than one problem. Starting there is a low risk item. You have to get the engine up to operating temperature first and then check inside temp of car before knowing if the rest of the components that supply inside heat are working. If you take the vehicle to an auto repair facility for diagnosis, they will aim their infra-red temp gun at the thermostat housing for a temp reading. That will determine a lot.
My 2003 Sonata does not "warm up" if it is just sitting. The car has to be driven 2 miles before the heat starts blowing hot air, before that it is just cold air. I can't defrost or warm up the vehicle in the winter, but have no problem with the heater after the 2 mile mark. Any idea how to fix this?