We walked back and forth when we stayed at RPR last week to check out Cabana Bay, and it is maybe 15 minutes or so. Upon exiting Cabana bay You have to cross a fairly busy street (there is a traffic light but no crosswalk which I found quite strange), then cut through Sapphire Falls then to RPR.
Note, if you are using Express Pass on your check out day as well as your check in day I think you will have to go back to RPR and print them out again, unless I am mistaken. You might want to verify this.