First, there is a difference between being stuck and being lost.
If you can move then you are not stuck and there is no need for you do /unstick
If you fall out of any path in the darkness, if the /unstick doesn't allow you to move, then the command to use is /die
If you are using the first person view, you won't be able to see your feet, even when looking down. So you can use the M key to toggle to third person view. Do not run since you don't know where to run to.
Once you can move, look around. It's the death realm, a dark and scary place. What you are looking for, is an exit. There is a magic portal (sparkling stars moving up) inside the mouth of a stone carved face.
The purpose of the death realm is to keep dead souls forever. Only the bravest are able to find a way back to life...at least in theory. In fact, as it is implemented right now (a search in this forums would have helped you), every curious explorer can find the way out.
In 4 hours, you should consider that you might have explored more than once the same places. Look around and ask yourself if there is a way to reach a place you did not yet explore. Not all places are reachable, but some are, including the exit.
Hundreds of people actualy found the exit by themselves. You can be one of them, or simply a follower of anyone you see passing by. If you can't follow one single running person up to the exit, follow each one until you reach a fork and see where the next ones are running to. Then look at the path they went to, explore it lookiing for a fork or the exit.