The walk back is a great skill to have that will compliment your obedience training. I use it as a fun trick, as a way to move my dog back if they come forward on a signal or position change cue and also drop on recall. It is a relatively easy skill to teach.
This demo is with my 4.5-month-old puppy Seven. I like to shape this exercise by teaching my dog to seek to putting his hind feet onto a mat that is textured a raised platform or folded towel, something that feels different than the floor surface they are on, when their back feet hit that target they know they have reached their destination.Once the skill is taught the mat can easily be removed and you just mark once your dog has backed up far enough.
To build distance and your dog being able to walk back in a straight line, I will lay chutes on either side with baby gates, gutters, etc. and have their mat at the end. I start at the end of the cute, walking my dog over the mat and reinforcing the hind feet on the mat. Then I start to move into the chute and asking for short increments going back until I have my dog going back thru the entire distance of the chute.
Have fun training this exercise!