Although I cannot speak for every author, I assume those that create an epic adventure begin with the world in which the story will revolve. The lands, the peoples of those lands, the creatures, geography and the plot will all revolve around where the characters are in the current narrative. When I began my epic tale, I began first with a map of the world. Then I sectioned the world into provinces, capitals and major cities, oceans, rivers, on and on.
From the map you are assisting the reader gain a visualization, and of time and distance. You may find yourself tweaking the map before your story is completed. I know I did. The hero of my tale had to set foot into all ten provinces to awaken evil forces and to draw out the free beings of the seven races to his cause. For in the last story of my trilogy there is all out world war. No one is spared, for the goal of evil is to destroy the first heavenly world created, but the last to fall. Now that I am writing the Chronicles, I will have nine other maps to make and nine stories to chronicle their utter failure, due to the influence of evil.
For me the map creating process was one of my favorite parts of writing this tale. I am not a literary genius, my grammar and sentence structure is poor. What I can do is visualize the world and the story. Then I go on an expedition to find the words that capture the world for the reader. I ratchet up the action with each book, to an exhausting finale, that has everyone in the world of Allivar taking a stand. They must take that stand, the last paradise is at stake. If you decide to take the chance of reading my tale, you will soon find yourself in a unique world, with unique characters, and an original story ten years in the making.
Enjoy the journey!