How do I get my Apple Watch to track the miles I walk?

The Activity App that is built into Apple Watch has this feature. Look for the icon on your Watch Face or you can add it if you have not already done so. Also, follow this link to read more on the Activity app, Use the Activity app on your Apple Watch - Apple Support.

In your case you are referring to the Workout app, follow this link here to read and setup for your outdoor walk: Work out with your Apple Watch - Apple Support.

If you're using the workout app as you say there are a couple of things you can try.

My Watch>Privacy>Reset Fitness Calibration Data

I'd try that first

Then reset your watch by pushing and holding in the side button and crown until the Apple logo appears.

If those 2 don't work I'd then go for an unpairing and repairing setting up as new.