A new Android Developer eview is now available, launching alongside the Fit SDK release. As the Developer eview page explains, this is a “special” build with support for the fitness program, so don’t expect to see new features or visual tweaks.
The preview build has been bumped to V81C, is available for the Nexus 5 the second-generation Nexus 7 only. Of course, it’s not suggested to use the builds for anything other than development purposes, as it is still incomplete contains bugs that need to be worked out. Then again, if you’re used to installing nightly builds of custom ROMs, you’ll be right at home.