Heater blowing cold to warm air.

1. It takes about 3-5 min and temp gauge is in the middle.2. Heater core hose on bottom is hot but top hose is just warm I can wrap my my hand on it and thats even after I have driven it for 30 min. What do ya think? I should replace first or check? 90 Mazda 626 4 cyl. Automatic. 69,000 mi.