So two things on this.
First... the need to determine all that ahead of time... that's what's creating the need for you to think so much about what's "realistic". I mean, it's a fantasy world? Does it have to work the same as ours? Does a desert form on the leeward side of mountains? Or does a desert form because hundreds of years ago, there was a great magical duel between two wizards, and the land remains barren to this day? There's no reason that we must portray a world like ours.
Second... if all those details aren't determined ahead of time, we're free to establish them as we go, and to come up with whatever justifications we need to at the time. This frees us up to allow something really cool in play to stand rather than saying something like "Oh, that'd be cool... but the ocean currents I wrote down in my notes mean that it doesn't make sense... so nope, cool idea shot down due to ocean currents."