My flight instructor would come at me with a baseball bat if I ever entered a runway without stopping at the hold short and asking clearance to enter the runway, or announcing that I was entering the runway at an uncontrolled field.
I've had to back taxi down a runway to the takeoff end both at an uncontrolled airport, and at a towered airport around Dallas. In both cases, I stopped and announced my intention to enter the runway. I have had the tower tell me to taxi to the runway end, and hold in the turnaround pad for another aircraft to land.
Twas a bit nervous about that one til I got to the end and say I could be almost 100 feet from the runway, and well before the displaced threshold. I even got told to line up and wait after the inbound aircraft passed me.
@ PawPaw's house - near KADS, Addison, Texas, USA