I think any kind of readiness implies preparation. Certainly, someone who is ill-prepared is not ready. Possibly in some cases it has more to do with alertness than with preparation for a particular activity. In such a case a person might be said to be "ready for anything". If I am "ready, willing, and able" then I am prepared for whatever might come my way. Of course, the definitions of "ready" that don't have to do with people do not apply.
(Readiness in the case of the military means that they are prepared for combat.)