Dev here - I had this idea while commuting some time ago and decided to code a little app around it: StepLock works like your standard app blocker, except that to unlock apps you have to walk a set amount of steps.
Just add any of your apps to the block list and set the number of steps you need to walk that day. StepLock will monitor your usage and won't let you use the app until you walked that specific amount.
The only thing is that the app use the "new" generation step detection sensors which are designed to be low power and have a minimal impact of battery, and are only present on newer-ish phones. Sorry in advance if the app is not compatible with your device.
