There is no escaping the fact that (re)developing a course for any environment takes time. Sometimes, however, it is not practical to redesign an entire course. And, because blended learning is extremely flexible in terms of delivery models, it is possible to incrementally increase the amount of out-of-class activities over the course of several terms. For example, the first time you deliver a blended course, you may develop two or three online or "mixed-modality" (i.e. a mix of online and face-to-face) modules, so that it is about 25% online. The next time you teach the course, you can build on this until you have the optimal mix of digital and face-to-face learning that meets your needs.
When developing a blended course, in addition to the regular planning of the course content and activities, you also need to produce digital content.