The answer provided by Newrot is a very good, simple, short answer.
The actual answer is very long and drawn out, and, subject to argument …
i will sum it up like this:
can you (personally) build a Rocket?
The answer is: Yes, you can.
Can you just build one right now? like with the current skill set and knowledge you have? No, you cant. (assuming your not an engineer of that type lol)
Can you build one with a ton of time and learning and understanding of the math and concepts and tons of drawings and examples and papers? Sure, that i do not doubt.
Building a massive MMORPG style landscape is much more then slapping down a 5,000km x 5,000km landscape and high-fiving your best bud thinking you just broke the world record in game creation.
Even games like World of Warcraft dont do this.
Now, to make it easy … i want you to think of a couple things that i am guessing, you missed in your MMORPG days.
What your thinking about is the transitions between areas. They are usually set between paths where you cant see behind where you just came from.
Early example: EverQuest “zones”. Would happen between cut out areas, and, have a turn then a long run, then a turn again. This would remove the place you were, and, make it so you cant see it go invisible.
WOW is the same way. They removed the “zoning” aspects and allow for smoother transitions, but, its still that way.
You can do as you wish, but, i will also tell you … that the task of a world that big, is not in your grasp. I am not trying to downplay your intellect, or, just say “you cant do it” for the sake of saying it. I am telling you because you will waste TONS of time and effort and you will destroy your learning curve (you wont learn shiz from this task). It will be painful lol. Very … very … painful.