Can I edit what is displayed the running scheduled workout screen?

Hi. The default screen is not customizable, but the other screens are - the ones you can navigate about by pressing the UP or DOWN keys.

Please check here for some guidelines:

support.garmin.com/.../

And here is a list of data fields you can choose from - the list does not contain absolutely all options since the manual hasn't been updated to latest software, but you will figure it out.

epix (Gen 2) Owners Manual - Data Fields (garmin.com)

Maybe I'm missing the point but heartrate shows when you make it a workout target

yes it would, but not necessarily the target for a structured workout - may be cadence or something else - in which the target workout screen will not show HR - because HR doesn't matter - the focus is something else. Which is point of the target screens - to get you to focus on what is purpose of the workout and not other features.

However the way around this is to build screens that do show you what you want i.e. I have a screen with HR gauge at the top and Cadence gauge at the bottom - so if its a cadence workout and want HR data I scroll to that screen and use that.

In my case, it is usually pace and HR together.  Pace will change over the course of the workout and is often hard to remember.  The workout screen will show me that, but then I can't see HR.  Sometimes they are as short as 20 seconds, so there isn't time to be swapping screens around.  I don't know why I can't either edit the workout screen or add the workout items to a custom screen. It just seems like a silly oversight.

Those fields are not available (unless I am missing them showhow).  That is the issue here.  You have a screen that you cannot edit, and do not offer the fields from that screen in custom screens either.  Why?

I don't think it is an oversight.

If workout focus is pace then your HR is irrelevent. Hence eliminate all non-relevant data like your HR so your can purely focus on the target for the workout i.e. Pace. If have to hit 4 minute pace your HR is irrelevant as some days, depending on weather, how your body feels it may be higher/lower - it doesn't matter - all that matters is 4mins. The point is you don't drop your pace if your HR is too high, or you don't increase your pace if its too low - the focus is 4mins.

If you want to target pace & HR set up a screen like that and use that screen instead. 

Workout fields are available for use. Only thing that different is the step time is going up instead of counting down. It is a category in the available fields

Mine are pace range and HR, and change as much as 10-20x during a run.  So, with all due respect, what you are saying is not correct.  I will have "pace xx:xx-yy:xx" and "HR Zone 2".  I know my HR zone, so all I need is my current HR.  Since it is changing often, it is hard to remember all the various paces on a particular run.  I have been living with this since Garmin removed the training peaks IQ app years ago.  Then I could tell it what I need to see.  It has been really driving me nuts lately.

I will look again, but I couldn't find them last time I looked. That would obviously solve the problem.

Are you using the connect app for this? That's the easiest way to create your data pages. I can't point out what to do precisely because mine is in Dutch. But it should contain step time, step pace, step speed