Heh. I double majored in physics and math in college because that was what interested me. I really didn't give a career much thought -- just majored in what I felt was interesting.
Then I did some graduate work in physics. That's when I decided that I really didn't have the aptitude for doing science as a career.
A stint in the Peace Corps made me realize that I wanted to be a teacher, and I decided that I wanted to be a high school math teacher. I went back to graduate school to bone up my math credentials.
While there, I considered a career in mathematical research, but decided it really wasn't for me, so I decided to become a college math teacher instead.
Right now I pretty much have the job I feel is right for me.
So, in a nutshell, this is what I did. I studied what I was interested in while in college. I didn't shut out any possibilies, and I changed my path a couple of times as I went through life.
Might not work for other people -- I only started my current career a few years ago at the age of 41. If you're not worried about getting a late start in life, then don't worry about taking a few different paths and making a few major changes along the way.