Let's split up the question a tad bit.
What exactly do I need to accomplish to learn and to master story-telling?
To learn is very simple, which you've already done through your prose. To master, though, is hard. And since there isn't a rubric to go by for mastery of storytelling(as far as I know), it's a difficult thing to pin down.
But to get better at storytelling, you should look at other stories. Go to a library or bookstore, or even a thrift store and grab a couple of books that look good. Read them, note what you liked, and then keep that in mind as you write your novel. But learning by example can only go so far.
I'd definitely join Scribophile as suggested by Chris Sunami, and maybe Wattpad as well to find a community that can give feedback.
If you want to, I'd recommend joining a tabletop RPG group as their game master. You do a lot of storytelling as a GM, and it really helps you come up with things on the fly. Very helpful for pantser writers. TBH, playing DnD as a Dungeon Master was probably one of the first things that pointed me towards writing.
Reading books which are written about it and developing stories with their techniques?
In my opinion, no. You write what you want how you want it. I might read one or two, but only to see what tips(if any) they put out that you didn't think of and then add them into your writing as you see fit. Kind of under this category, if you see an ad for Cat Rambo's Academy for Wayward Writers(or something like that, I think) on the site, click the link. (Or just use this one) There are online writing classes there that help a ton. I took one on writing fight scenes and it helped tremendously.
Affiliation disclaimer: I took one class, no other affiliation
Writing novels with no concern for formulas with dependence on my power to imagine?
Do this one. Throw the rulebook away and write what you imagine. You might be bad at first, but there's always a second draft. Just write what you want. Formulas are something for 3rd and 4th drafts, if ever. You wrote it, so it's whatever you want it to be. If you are writing to publish for money, though, then formulas would probably be a good idea.
Is it necessary to build up measure through practice?
Sadly, yes. Reading and playing tabletop RPGs will only get you so far. But since you've done prose, you have a head start. There isn't really a set strategy for getting better through practice. Just write. See what happens. Maybe try a NaNoWriMo every few months. Look for feedback on your writing, implement it if it's a valid suggestion, and keep looking for help to improve.