Perhaps you would have spent a few years working in an ad agency, and then made the decision to go freelance.
If you didn’t take the agency route, then you would seek out the small number of books that had been written about copywriting and freelancing.
Either way, you would continue to learn – about both your craft and freelancing – at a fairly slow pace. You would learn from a very small handful of authors. You would learn from your clients. You would learn by reading the occasional article in a trade magazine. And you would learn by studying the successful work of your peers.
As for taking in-depth courses to speed things along, no such luck. As far as I recall, there were no courses or programs devoted to copywriting or freelancing back then. Just books and an occasional magazine article.