Watch series 3
When I walk, I set the walk app. I walk 2 miles for 50 or so minutes. Yet the watch tells me I’m only done 20 minutes of my 30 minute exercise, not closing my exercise ring
Apple Watch Series 3, watchOS 6
According to this, Get the most accurate measurements using your Apple Watch - Apple Support exercise is counted as a "brisk walk" and it has to do with you moving your arms, and increasing your heart rate. There is a section on an alternative measurement, and it has a link to another support document if you are not able to swing your arms, such as when pushing a stroller. See if that helps.
