OK, well first of all there will be no more courses on leadership allowed on Udemy.
Just kidding. Check out my home page and you will understand why.
Seriously, I think you need to consider the difference between effective college lectures and effective online course lectures and course structure. Don't take it from me. Read the research: https://community.udemy.com/t5/First-time-Course-Creation/Lecture-Effectiveness-The-Research/m-p/141....
Your intro/promo lectures should be about three minutes, max. Course lectures should be less than five minutes, although I violate that sometimes. I can promise you that prospetive students will not make it through that promo. Check out Angela Yu's promo lectures, and other top instructors. There is an obvious reason why their courses sell.
Also seriously, welcome to the club and good luck with your courses.
It's easy. Go into your course page and on the left click on "curriculum." For each lecture, on the right there is that little down arrow. Click on it. You have toggled "Free Preview." Un-toggle them. By default they are not free previews.