If you already paid to unlock everything in The Walk before December 2019, then you won't need to pay or subscribe again.
However, that purchase is tied to the App Store account, or Google Play Store account that you made the purchase with, and it is not possible to transfer that purchase between platforms.
Purchasing The Walk individually also does not grant access to Zombies, Run! or the Zombies, Run! 5K Training App.
If you're an Android user who bought the app before November 2017, read this document.