That's admirable, I couldn't read that much!

I'm wondering what Campbell did achieve by doing all that reading and what made it the most important period of his scholarship and study.

The 10,000 hour figure (which is just an arbitrary number, not an absolute rule according to Anders Ericsson[1] whose research Malcolm Gladwell referred to) implies you can reach mastery with a lot of practice, given you do the right kind of practice.

Mastery is the ability to perform a skill, it's not about knowledge. You need knowledge to perform, but knowledge without being able to perform does not imply mastery.

[1] https://www.amazon.com/Peak-Secrets-New-Science-Expertise-eb...