What if you use gps spoofing apps to easily simulate walks and distances ?
It uses the builtin fitness api that tracks your steps rather than GPS (I checked the information in the play store).
You can still cheat the system but I'm not sure how likely that is.
I'd guess as long as people are having fun with their bit, it doesn't matter if there are people trying to cheat the system - especially if there are other protections in place.
There are a lot of wagering/game apps that come and go and I think the op is only making the app for certain countries atm too it seems.