Hi,
as I did not find any satisfying guide or method on how to use the Garmin watch for a Run-Walk-Run training session with the Garmin trainings plans of 'Coach Jeff' I did create a HowTo that you can find here How-To Garmin RWR FIT-file
This requires a little bit of computer skills (using the Garmin FIT-SDK) but you might also be able to find other tools that allow you to edit FIT-Files.
I hope it helps someone!
Have fun - Guhl