The overflow tank is only to keep air out of the system. Since hot liquid (and gases) expand it has to go someplace or the system would overpressurize (bad). Then when the car cools off the coolant contracts sucking coolant from the overflow tank. If the overflow is empty it will suck in air instead of coolant, filling up your cooling system with air.

You were/are getting heat because now coolant is flowing everywhere it's supposed to be. Believe it or not your car's interior is heated using the coolant, which is why most people tell you if your car is overheating turn your heat on full blast. And you may not have all the air out of your system if you didn't turn on your heat when you filled your car with coolant, since there was air in the heater core. Your cooling system was probably almost bone dry, which is why you were overheating.

When was the last time you checked your coolant or topped it off? This is why basic maintenance is important to follow. I usually do this:

Once every other day or so:
1) check for flats
2) check for odd scratches or rust spots

Once a week:
1) check air pressures
2) check oil levels

Once a month:
1) check coolant levels
2) check washer fluid
3) check battery terminals
4) check engine area for any weird fluids or anything leaking

I'd still have someone do a pressure test just incase to see if your headgasket is gone. If you let the car overheat too much or too long you could have warped the head which is also bad. Honda engines are made out of aluminum which is great for saving weight, but bad since they are delicate.