I play at Fortnite now and then Arena a little bit more and will soon be Division 5

PS: Should you actually play a complete session ONLY arena or normal rounds, and how do the pros keep up with arenas for so long?

And I'm a mediocre good PC player, but arena can improve you too, right?

Of course you improve with arena. It depends on how many rounds you complete each day. Of course, one round is not enough.

Yes 5-10 or more…

Is good.