why does my activity app show only a certain percentage complete on a walk for example? I assume I saved a goal for a walk at some point, but can’t change it. My walks are different everyday.

Hello.

It seems like you set your walking goal inside the Workout app on your Apple Watch.

And since then it shows the percentage of your walk according to the set goal. You can change it in Workout app by tapping three dots in the top right corner, or just choose "Open" from there and walk however you want!