I guess that's part of the fun of it being a hobby. It flows like a stream of consciousness. It morphs and changes and you get to mold it into the experience you've always wanted to build.
As fun as that is, progress can stall. I mean completely stall. That's never a good feeling. Even coding a dozen lines to allow a new thing is work and I've decided to employ an extremely loose definition of "sprint" to my game development.
My bucket size will continue to vary in size from sprint to sprint, but I will save out my project and create a new version that's playable every Friday. I'll also include a change log of the things that we added, removed, or modified.
I can only hope this kind of rough regimen will keep me productive.