I think there's a small fallacy here, that is that many people 'decide' what they are going to pursue as a career.
Of course, I can only speak to my own experience. I started my career back in the 70s as a musician. I had a lot of interests, but at the time music was what I was 'good' at without trying too hard. A few years into college (on a music scholarship) I realized that a career in music didn't jazz me at all, and I drifted into the military (Air Force). There another interest, electronics, was useful in that it landed me in in a small lab in NY, supposedly to fix broken gear, but what ended up being a four-year stint doing primary materials science research. There was a pretty good engineering school nearby and so I picked up a degree in electrical engineering.
When that was done, a few coincidences led me to magazine publishing, and I was Editor in Chief for a few years of a large electronics hobbies magazine. Money called, and I left to do course development at Digital Equipment Corp, learning and writing about operating system internals and networking. Another degree, this one in CS.
Then a marketing job. Then a decade as a consultant. Then a director at a research firm, and a director at a startup. Founded a small publishing company doing 400+ fiction titles. These days I'm a professor in computer science at a prestigious university.
I didn't plan any of this, really, and my advice to students who ask me this question is to be open to every opportunity. Life just happens, and things come up, and you have to be ready to jump. The but first part is that when something interests you, let it consume you. Dig deep, be the expert, but when something else catches your eye, be ready to move on.
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