Apple Watch pop up

That popup would generally only appear if the Watch determines that it appears you are no longer working out. If you are finished with your workout, you would hit End. If you are just taking a short break, then Pause. What exactly are you doing that the watch would assume you may be done. Or, is it that you are not working out at all and may have tapped on a workout by accident?

That is correct. If you are just looking for a particular time period, you can look at individual statistics during the day, broken down by hour, or you can use the workout for a walk and open goal. That should not necessary prompt that pop up. Otherwise, you can provide feedback to Apple that you would like to see a change. www.apple.com/feedback/watch.html

I am not aware of any way to disable it. If you are just walking and want to track mileage, just use the Health app, it is tracking your steps and distance without using a workout setting.