An insight I've had here is to write the code the same way I would have as if I'd done red green refactor tdd, but then add only the essential unit tests at the end. Works very well and is much much faster, but obviously you'd have to have done a lot of red green refactor tdd previously in your career or on side projects to really know what testable code looked like. Reading Working With Legacy Code by Michael Feathers may be a shortcut to that knowledge as well.