Honing your judgment
The HBS [MBA program] is a wonderful experience but it does not and cannot prepare you 100% for general management. If anything, HBS erroneously teaches you to look to the data and analyze it. In the real world, data is often lacking, wrong, or contradictory. Answers are rarely clear-cut based on data. Financial analysis of investment opportunities based on ROI analysis, for example, rarely answer the questions of capital allocation. The numbers come out any way the sponsors want them to.
What is key in being a GM is to develop and hone your judgment of people, behavior, and situations. This is why it takes a number of years of experience to graduate to a GM job in a corporation.
Seeing the big picture
One of the keys is to maintain a general management perspective even while you are accumulating functional management experience. As experience hones "instinct," it is critical that you see the big picture even if your current function is a vertical one.
Learning from a mentor
In looking for a job that will prepare you to be successful as a general manager, hopefully as CEO, look for an organization where you can have a mentor that you can (a) look up to, (b) learn from, (c) lean on, and (d) that will take a personal and professional interest in your development.
Apprenticeship has value. Those who shoot up quickly need extra special humility and an openness to listening and taking advice from subordinates. No one knows everything.
General Managers are responsible for everything.
If they haven't served their time in the ranks, they can make big misjudgments if they aren't willing to listen. However, successful general managers come from a variety of backgrounds and experiences. I'm not sure that there is a general rule. So much depends on the individual's personality as well as his or her experience.
As in parenthood, you can read all about it in advance and still not have any sense of the actual experience... but after you've been doing it for a while and you go back to one of those books, you may understand how much wisdom really was there. In the same way, becoming a general manager will be nothing at all like what you expect. But after you've done it for a while and look back on your preparation, you may be surprised at how well prepared you are.