TDD is not Unit Testing; at this point, most of your code is written and needs Unit Testing as you "fix and tweak" to help avoid breaking things as you fix, and confirming that what you intended is what will actually occur.
Nowhere did I say TDD was unit-testing. But unless you have a magical wand of 'instant unit-testing', I think TDD is the best development practice for implementing unit-testing gradually rather than attempt to do something akin to the doctrinification, where features were put on freeze to convert the codebase (mostly).
