I do blame Google a little bit, as in you can't just give someone an app-store link, they join your test and can play.
Having to invite each tester per email, or have them join a Google Group really hinders things. I did try making posts on Reddit to find testers, got 1000 views and barely anyone signed up :-/
As for marketing, sure, I could always fall back to that. But it's a fully free game, I'm not expecting to make any money on it. It does have a "Support the developer" option, but those usually don't work well at all (no reward for it, fully optional donation).
Hell, I made the game because I wanted to go on more walks myself, but then I spent the last months in front of the PC actually making the game instead of walking, so that backfired :D