If it's on the "H" chances are you ARE overheating. You're not going to start smoking until you've either warped the head and start burning coolant or a radiator component gives up and cracks. Easy way to check though (aside from looking at the gauge ), simply pull over and look at your radiator fluid catch tank. If there is a lot of fluid flowing from the neck into the tank you are definately overheated.

Anyways, since you just had a coolant system repair, it might be a simple case of low coolant or air in the system. Air in the system though would really only happen once though since the bubbles should evacuate once the car has been heated then cooled back down.
If there is plently of coolant in the system and the car continues to overheat I'm going to blame the pump since you said you were able to drive around for 30minutes without it going down. This is assuming of course that you weren't city driving or in stop-and-go traffic. In my experiences, a bad thermostat will cause the car to take forever to heat up since they usually get stuck open when they die though it is possible for them to be stuck closed. You can easily check for thermostat opening by touching the upper hose and seeing if it is hot or not. If it's hot, the thermostat is opening.