Mapping every corner of the world in 10 foot increments would be a ridiculously hard task. Instead, map the starting area and a few points of interest you know you'd like to use, plus describe major landmarks visible from the starting point (dark mountains to the east, ocean to the west.) This works sort of like a MUD/MOO: when the players choose to travel away from the starting point, they will describe what they do in terms of landmarks: "We follow the road out of town to the south for about a mile, then we veer off the path towards that hill with the outline of a crumbling ruin on it."
Also create one-sentence summaries of major conceptual groups at various levels of detail, starting at the top (how many continents are there, and how would you characterize them in one sentence?) and drilling down from one of the top-level groups into a sub-level, picking one of those as a start and drilling down further. For each item labeled "starting" (the starting continent, starting kingdom, starting province/barony, starting town,) add three to five one-sentence statements about what the players are likely to see or what is likely to happen to or around them. I call these "factoids".
Your process looks like this:
- there are five continents, here are one-sentence summaries of what they're like... here are 3-5 factoids about the starting continent.
- there are seven major political groups on the starting continent, here are one-sentence summaries of those...
- starting kingdom has three to five factoids, like "king enforces loyalty from subordinates with combination military/secret police that wears magical bone armor". We now know something about the king (he's distrustful, he spies, possibly tortures citizens,) and we know something the PCs might encounter (secret police, wearing bone armor,) and something about what might happen in the background (PCs hear that their favorite barkeep is gone because he was hauled away "by the bone folk".)
- here are the major forests, mountains, and other geographical features, with one-sentence summaries of what they are like... and here are 3-5 factoids about the starting area.
You now have sketchy details of the entire world, with more and more detail as you get closer to the starting area. When players ask questions about something not in the immediate area, you improvise based on the one-sentence summaries or bundles of factoids. When players decide where to go, it's true they could go absolutely anywhere, but either they will pick something you said that intrigued them ("I wonder what that continent of civilized winged monkeys is like?") or they will head towards a visible landmark. Thus, all you need to know is how to improvise a journey between two points.
Using your sketchy knowledge, you decide what kinds of areas are between the adventurers and their destination. ("Oh, they have to head through the Ghost Woods to the Titan-Shattered Hills, find the safe pass that is still defended by rangers, reach the decadent sea port, sail across the sea of knives, make port at the monkey kingdom's trading post for outsiders.") You thus divide the journey into waypoints.
Add three to five factoids about the general nature of each waypoint/region ("The ghosts of the forest are gnome martyrs seeking vengeance against a long-dead empire of ghouls.") Also add visible landmarks for each waypoint you now know the adventurers will travel through, so you can describe the world around the PCs (MUD-like) as they travel from waypoint to waypoint; as they pass through each waypoint, the party may decide to veer off course, but it will be towards something they see or know about, like one of the landmarks.
You should also know about how long it takes to travel through each waypoint, normally. This lets you decide things like how often to roll for random encounters, assuming you want to play out the entire journey. You can either customize the random encounter table itself, or modify generic random encounter rolls on the fly; either way is based on the summary of the area and the factoids of the waypoint.
Create a random table for each region/waypoint of the types of things found in that kind of terrain, like "forest stream"; again, modify the results of each random roll based on the relevant factoids/summaries of what this particular forest (terrain) is like. That stream is going to be different in the Ghost Woods -- maybe people hear vague whispers if they stay too long near the stream, maybe there's a chance of seeing dead faces peering back at you when you gaze into its waters.
When the party reaches the destination, if it's a new kingdom or continent, fill in one-sentence summaries for subdivisions of the location, and add three to five factoids about the new starting location.
If the party chooses in transit to go somewhere else, like towards one of the landmarks along the way, or towards something you improvised in conversation during an encounter, or following a random stream or other feature rolled from the random feature table, you now have a new destination; break down the journey into waypoints and begin the elaboration/improvisation process again.
For assistance in filling in details, you can use and reuse geomorphs of various kinds, rotating them and rolling for random variations.
At no point are you mapping anything that you think the party might visit without them indicating that they may in fact go there. You're mapping stuff as they visit, and filling in new areas with summaries/factoids/landmarks as they approach. This reduces your workload to what is necessary, rather than what you think might be needed.