In teaching theory, explaining something to someone else is learning it a second time. You have to genuinely understand it before you can teach it. And if you get it wrong in front of the class, that is fine too, because that is the moment you discover you had not understood it.
What my study day looked like
My most productive period ran like this.
Four, half past four in the morning, I studied the material. Around five or six I took a break, showered, ate, got ready. Eight o'clock I was at the school, doing a bit more.
By nine, when everyone else was filing into class, I already understood the whole lesson. Class then felt like a discussion, a conversation among people. I was verbalising, saying out loud the things I had learned that morning, rather than meeting them for the first time.
By three in the afternoon, when everyone was flagging after a full day, and with revision still to come that evening, I had already been through the material two or three times. So it stayed.
If there was time left I did a few more exercises, and that was that. Evenings were for other work.
Why this works
The difference is not more hours. It is that each encounter with the material is a different kind of encounter: reading it, saying it, then revising it. Three different modes on the same content, rather than reading the same thing three times the same way.