So here I am blathering about all these landform shapes I "see", even though all you drew were the highest mountains. How? And how can you learn to see it? At the most basic, you've depicted highest and flattest. You know how to see topography shapes based on contours? Can you? Even without a crazy-intricate set of contour lines, look at a map with a well-defined set of (plausible) rivers. That net of lines indicates all the locally-lowest spots. The blanks *between* the river lines show a set of ridges. On steep terrain, sharp ridges. In flatter terrain, low mounds or barely-discern able rolling swells.

Further clues to the land can be inferred from how tightly packed the lines of the river net are. Sparse looks a little flatter and/or drier. Since you're making this world, you can decide how small of a stream you'll show. Continental maps won't depict down to inch-wide rivulets. Rather you might stick to only the parts of various branches that are navigable... Whether by rowboat or by steamboat or by supertanker is up to you. Face it, way up in the mountains all the streams will be pretty small. At most sketch a few of those in - maybe just the ones watering settlements or defining borders.

Rivers don't typically just flow *into* mountains. But they might pass *through* mountains or other high land. Maybe a pass or a water gap or a canyon permits a low enough passage for flow.