Hello sophie3210, my suggestion is close to Uncle Bob's.
"Rtn" is a standard abbreviation for "return". "Distance rtn" has the same number of characters as "distance A/R". I'm pretty sure, but not certain, it would be understood. Perhaps someone else could comment.
'Distance Rtn' sounds really awkward (unless the 'rtn' was in brackets, like 'distance (rtn)'), but I'd recognize 'Rtn' means 'Return' right away. It's clearer than my RT suggestion, which can be interpreted differently depending on who uses it.