The World as We Know It

The world in my mind is immense.  Larger than any planet we know, perhaps the size of our sun.  For whatever reason, this world supports life, and has a gravity and atmosphere similar to our own in spite of its size.  The reason for this will be explored in another adventure.  This is a world so immense, that one could start walking at birth, and still never reach the other side of the planet before succumbing to old age.  The entire planet is interspersed with various civilizations, with vastly different environments, histories, and cultures.  Because of the immense distance separating them, most of these civilizations know little to nothing about those outside of their cluster of cities and towns.  Some of these clusters are made of dozens or even hundreds of cities, spread out over huge distances, as vast and populated as our own world.  The next group of cities over could be anywhere from a few months' travel to decades away.  Imagine a solar system full of hundreds of inhabited planets, each with dozens of cities, all combined into one immense planet.
A world so immense is obviously home to vastly diverse environments and physical settings.  Oceans as big as our world, mountains so high the clouds merely encircle their base, and valleys that disappear into the darkness of the planet's insides spread across the landscape.  One can find in this world cities built in the canopies of trees that stretch a mile into the sky, or farms stretched across plains that disappear into the distance of the horizon, or plateaus housing each their own unique ecosystems.  The diversity is immeasurable, though these writings will help to explain what has been explored of this world.
The people and creatures are perhaps even more diverse than its landscapes.  People of all different races and cultures dot the surface of the planet.  Most will never know of one another, but some individuals devote their entire lives to traveling, exploring, and finding ways to move more quickly across the landscape to better realize its inhabitants.  Civilizations and people form relationships across the planet with others they come into contact with, forming alliances or waging wars, or trading ideas and technology. 
Every part of this world is in a constant state of growth.  This is the world as we know it.