You might have a problem with your engine thermostat? Does the engine take along time to warm up also? Failing that you might have a problem with the duo valve failing to open altho they normally fail in the "on" position in the event of complete failure.
I'm not sure if THE W126 has a small recirculating pump in the heater circuit like the W124 but this might have failed /fused and might explain the slow warm up?