Longevity of Ad Careers

In my advertising class, my professor has brought in many people to speak to the class about being in advertising. These people speak about what it takes to make it in advertising and they discuss how they get to where they are and what they do now. Im extremely interested in what they do now and some of their jobs are awesome. What I am mostly concerned about is how they got there. Most of the people that came into class were not ad majors when they were in my position. They were some other major and then fell into advertising somehow. Also, it seemed none of the people who spoke had kept the same job since college. Almost all of them have moved companies and switched their jobs completely. This concerns me mainly because I am taking a different route and I am not a big proponent of change. I like to know what I am doing in the future. Long story short, I dislike uncertainty and there seems to be a lot of it in advertising.