The research that Gladwell referred to in his book Outliers was done by Anders Ericsson, Ralf Krampe, and Clemens Tesch-Romer in Germany, and focused on violin players. It claimed that the best violin players spent 10,000 hours practicing by their 20th birthday. Since then, the findings of that research (and Gladwell's use of it) were challenged many times, including by the authors of the original study.
My focus here is not necessarily to challenge the rule, but to show how you should spend your time tin order o be the best in your field--whether or not you follow the 10,000 hours concept.
First of all, recognize that different disciplines require different types of practice. The practice required to be the best college professor is different than the practice required to be the best Olympic skier, and different than the practice required to be the best CEO, and so on.
There is no doubt that the more you spend your time in one area, the better you are at it. The experience (or learning) curve is one example of it. The more you do the same thing, the faster you do it, and the less it costs to do it.
But, it can be difficult to figure out what to spend your time doing. In my experience, there are three areas in which you should spend your time if you want to be the best at something. They are highly-related, and all three are required.
You must know almost everything there is to know in your field. I was a shooter with Olympic aspirations. But when I met an Olympic gold medalist shooter, I found out how much I didn't know. That's an important piece: you have to spend time in learning what you don't know. Don't leave any stone unturned. Broaden your perspective as much as you can. Learn by reading, watching videos, and learn from people who are great in the field. Spend your time wisely. Evaluate the quality of the material you are about to study before you delve into it. Focus on learning from thought-leaders in your field, not from just anyone who decided to teach something or write a book. Your time is too precious to spend it on less-than-adequate learning.
Knowing is not enough. You must put to practice what you learned. This is also the time when you generate value from your area of expertise. If you are a violinist, you must also play violin--a lot. It's not enough to know music theory. If you are a shooter, you must shoot a certain number of rounds every week to be the best. Likewise, it's not enough to know everything there is to know about management theory, without practicing it. Through practice, you are building muscle memory, brain capacity and skills.
Once you reach a certain level of skill in your field (this will be long before you are the best at it), start teaching others. Teaching forces you to learn even more, and build more experience. After all, you must be prepared to answer challenging questions. When you teach you are also challenged by your students and are pushed to defend the way you see things, or even change your thinking. Teaching does not only happen in the classroom--it can take place through writing books and articles, or even blog posts. Finally, teaching also helps others become better, not only you.