There's a setting for that...
- go to the Walk 'app' (this icon for your walk) on your Charge
- don't tap it to Start but scroll down - you'll see various things
- the setting for Auto Stop is just below that for Auto Start
- both of these are below GPS, which lets Walk use that if you've enabled it elsewhere
- lots of little switches, very easy to move them when you don't want to
- so once you have what you want, use the button on the watch to return to 'neutral' -- the time watch face.