You can change/customise the data fields for any activity.
When you choose a workout you'll be prompted to pick an activity - or if you do it the other way round you'll pick an activity and then choose to do a workout.
If for example you choose the run activity then it will usually be set up to show pace/hr etc.
You can change the fields that are shown by following the instructions in the manual as per below
http://www8.garmin.com/manuals/webhelp/fenix5/EN-US/GUID-638CD68D-11B0-4D9C-B8B7-E28D15EC4566.html
Hope that helps you!
Right I understand your query now. I've tested this and can confirm this works.
You need to go to Settings -> Systems -> Format -> Pace/Speed Preference -> Running
If you change the setting from Show Pace to Show Speed then the alerts will be speed based
Understand what you need and unsure how you might do that.
I'd be more concerned at 12kph being a pace of 9:03 min/km as 12kph = 5 min/km