IMMEDIATELY check your coolant level in the radiator. Keep an eye on it. Do not let it go dry.
You may want to consider a flush-n-fill, with a new radiator cap and new thermostat. Also, still replace the hoses. There may be an air bubble, so try and burp the engine when you re-fill. And read this post for more info to make sure the heater control valve works.
Supra MkIII - Heater Problem Solving