on my 99 Exp 5.4L -

I recently replaced my oil filter/radiator gasket. (the one attached to block under the oil filter) which caused a loss of coolant.

So I decided to unscrew petcock to drain the rest; not much came out. It was like the petcock was plugged.?. I finally topped off with only 2 fresh gallons of coolant.

In doing all this, is it possible that I got air into the system (like ive read on a post here somewhere)? If so, how do i get it out?

I ask because now my upper radiator hose never gets Hot to the touch and I dont feel it under pressure as I squeeze it. Most importantly, I am getting only lukewarm temperature out of my heater in the front and rear of vehicle.

Bottom line - did i screw something up while opening up the coolant system?
I do appreciate all your help.
P.S. - the temp gauge reads normal under full operating temp...