I keep fictional worlds in my head much like I keep the real world in my head - by keeping track of directions instead of keeping track of the map. It's fun. A good way to keep it less abstract.
For example, lets imagine you're in London right now. Get a compass or use google maps to figure out which way is east. What lies in that direction? If you're in central London, traveling directly east means covering a bit of city to South End, then there's the sea, quite rough and harsh this time of the year. Gray waves reach high, tossing the sailing boats and the fishing vessels. The ferries pass. The seagulls pass. Then you reach land in the Netherlands, near Amsterdam. Old city architecture. The canals. The fields. Lots and lots and lots of open agricultural land. Forest. Fields. Germany. Large cities and dense networks of roads. Old cities. Some abandoned and ruined. Occasionally ruins froma ll stretches of human history. Keep going. More fields. More forests. You're entering more mountainous terrain. Lakes. Rivers. Poland. Ukraine. Russia. Russia. More Russia. No need to picture them in great detail, just get a sense of what lies in the direction of 'east' of where you are standing right now.
Keep going until you go around the globe and return to your original location. Then look at your compass and pick another direction. Map out the world in relation to where you are. Note the direction of various lands and landmarks in the world. Look into the direction of Africa. Point towards Australia. Point towards the north pole. Actually be aware of what lies around you and where you are in the world. You're tiny. You're immensely tiny, and the world is huge.
After a moment of doing this, imagine yourself in the location of your story. Not in the middle of an abstract map, but a location. If your story takes place in a city, imagine yourself on a rooftop. It could be day or night, summer or winter. There is a compass in your hand. Locate east. Then off you go. Explore the major landmarks of the world, continent or planet until, when you return to that city and that rooftop, you're able to look at the compass and point in the directions of the landmarks you've thought up.
That's what I do. It's fun. I used to start with making maps and plan according to them, but that's too abstract for me. I like to play around with being in the middle of things and get a sense of distance, space and landscape, just as a game. The reason I do it this way is because to be honest, I don't want to explore maps. Maps look very boring. I start with exploring and then I make a map of the interesting locations. This means I get to look forward to returning and explore places in more detail.