If you are a new teacher, this is a concern. No one wants to get to class and find out they can only fill half the time.
There are several things you can do, all of which require work on your part. (Sorry, there are no easy answers.)
Practice out loud.
If you are going to be giving explanations, lectures, or general information, practice saying these things. It will let you know how much time you are going to take. It may also help you determine a better way to say it.
Have activities and exercises.
When you are working on a topic, always have more activities and exercises.
If you think the class will have time to do two activities, have four. If you think the class will have time to do one, have three.
Sometimes I will plan something that takes too much time to accomplish. I can usually have them finish at home or I can shorten the amount they actually have to do.
But sometimes things I think will take a while are a sail-through for the students.
Having activities and exercises helps emphasize whatever you are working on. And it keeps the amount of time in class full.
Also remember that having students work more on a topic usually enforces that topic. Students will remember 10% of what you say and 90% of what they do.
Have a related writing project.
If you are discussing neo-classical literature, you can ask the class to write a paragraph (or more) reviewing the major points discussed in class.
If you are lecturing, you can ask them for the three points they best remember from the lecture.
If you are having them do a reading, you can ask them to summarize the reading in a paragraph.
Have group discussions.
If you have just given a lecture, have them get in groups and discuss what they remember of the lecture. If they know more on the topic, they can share that, too.
Have a quiz.
Ask them to apply what they’ve been learning for the last few classes. For example, if you are reading Swift and have discussed satire, ask them for examples of satire in Gulliver’s Travels.
Or if you have just done a reading, ask them to identify a theme and defend their identification.