How to teach from someone else's material? This semester I've been teaching a few hours for another professor (I'm a Postdoc). The idea is that I get some teaching experience while he gets a few precious extra hours for his research.

His slides didn't come with any notes, and he hasn't been very responsive in answering my questions about exactly what he said when he delivered the content. I revised the material and edited the slides in preparation. But, after the first hour of class, I've covered about half the material, which was supposed to last three hours. So, I guess either I've missed something implicit (not on the slides) or I've just not paced the material well.

Short of practising in full, how does one take, adapt and own somebody else's lecture materials?
In addition to the slides, use an accompanying textbook to prepare the lecture. This will give you additional context material to fill the lectures, as well as small exercises you can go through at the end of each session.