You start. That's all. Just... start. Is one unit test doing anything? Yes. Absolutely. It's giving you 1 more unit test then you had to begin with. Once

You start. That's all. Just... start. Is one unit test doing anything? Yes. Absolutely. It's giving you 1 more unit test then you had to begin with. Once you have a test, you can write another test. You can refactor a method (And write tests). You can then refactor the calling methods. And so on. "It will be a mammoth task" is only a good reason not to do something when it's either discardable in the near future (And not "When we start on V2!"), or if it's the canonical 9-woman-1-baby situation. If partial improvements give value, do them.