If you couldn't make it to Google I/O, and thus couldn't get one of the first Android TV units as part of the developer swag, you can still start developing your apps for the platform's retail debut later in 2014. Google has included Android TV modules in the official Android SDK, underneath the Android L (API 20) package. That includes an emulator specifically for TV, so you should be able to build and test apps without any extra hardware. There was a similar emulator for Google TV.
That being said, an emulator usually isn't an ideal solution, since it will almost certainly be slower than the ADT-1 developer hardware. Considering the higher demands of the media-heavy apps that Android TV will feature, that could be a big hurdle for developers. The more processor power and RAM available on your development machine, the better off you'll be, but there's no avoiding the performance hit inherent in emulation. It will also be difficult to simulate the input of the controller included with the ADT-1.
Developers should be able to modify existing apps to run on Android TV, but depending on your needs, it may be simpler just to start from scratch. To set up the emulator, head to this page, download the latest version of the Android SDK, and follow the setup instructions.
Thanks, TheManii and Ruben Roy!