It's going to take more than 12 hours to actually become proficient in the material.
I think the core ideas are actually rather simple. In my opinion, the bulk of time studying calculus is memorizing _algorithms_ for conducting differentials and integrations and which algorithms to apply, say, to find the area of a solid; which is arguably a great fault in mathematics pedagogy.
The bulk of my time was spent doing practice problems until I was sure I understood how to solve them - so that I was sure I knew when to apply algorithms. Also some brutally long assignments.
Totally agree. Watch a course like this, read something like Calculus Made Easy by Silvanus P. Thompson, and then work through all the problems in a book like Calculus Workbook For Dummies. Watch Khan Academy to fill in any areas where you're struggling.
Similar opinion here. Seems like there are only 2 or 3 key principles that must be known to understand calculus. Most anyone could probably understand limits and the Riemann sum.