One of my favorite things when cracking open a new book is to pore over the maps. It kicks off my imagination. I wonder about how some of the details are going to be used in the story. Undefined edges recall the Age of Exploration and those cryptic notes, like "Here be dragons." The one thing that bugs me (yes, it's mostly because I love geology and got the degree) is when topography makes no sense.
Plate tectonics is the widely accepted theory of how continents move across the Earth. On the surface are basically two types of material: oceanic crust and continental crust. These both "float" on a layer known as the mantle, which is liquid due to the weight of the crust (the core is below the mantle). A cohesive mass of material (which can be made of both oceanic and continental crust) that all moves in the same direction at the same rate, is called a plate. These plates move very slowly with both direction and rotation. How these plates interact affects almost every physical feature on the Earth. If this theory functions on your planet (which as a reader is always my assumption), there are infinite ways to arrange your dry land, but only a few ways to create a mountain.
Continental crust is less dense than oceanic crust. Consequently, when the two meet, the oceanic crust is pushed down (subduction) at an angle. It is theorized that at depth this subduction causes melt, putting pressure on the underside of the continental crust (vulcanism), causing uplift. This is one of the primary methods of mountain-building. A modern example of this is the Cascade Range of the Western US and Canada.
It seems that when two plates consisting of continental crust meet,
neither wants to subduct. In this case, the land masses smash together and
form some spectacular mountains. The Himalayan mountain chain is an example of this kind of collision.
In a place where two plates consisting of oceanic crust meet, the denser plate (not all oceanic crust is created equal) will be subducted. Again, at depth this causes melt, inducing vulcanism and creating what are known as island arcs (like the Aleutians). When this resulting oceanic crust is later subducted beneath continental crust, the islands are essentially scraped off, slamming into the surface material (making mountains and causing some lovely metamorphism). This is thought to be how the mountains of the Eastern US formed (the Grenville Orogeny, Taconic Orogeny, etc).
The final basic way known to form mountains is through hot spot volcanism. Some volcanic island chains are formed in the middle of a plate (like the Hawaiian Islands). The thought is that certain given areas of the mantle are significantly hotter than the rest. These "hot spots" burn their way through the overlying material to reach the surface, each building a volcano. As the plate moves over a hot spot the route to the surface outlet becomes inefficient. The magma then has break through at a new spot in the crust to fight its way to the surface. The subsequent chain of mountains (or islands) then shows the motion of the plate over the hot spot.
Woah, that was loads of information. Feel free to look any of that up on Wikipedia for diagrams and such. Most of the technical stuff is much nastier to read.
Following this info dump, you may rightly ask, "How the hell does this relate to my maps?" Well, I'll tell you. On a continental scale, mountain come in chains. These chains tend to be linear features. It would take some real finagling to make a continent which has a cross or a ring of mountains (like Mordor). Similarly, one mountain in the middle of a vast plain is really unlikely (still looking at you Tolkien*), unless it's a volcano. Even then it should probably be close to the shore (sorry Mount Doom). By patterning your fantasy world after nature, you can allow it to grow on its own.
Lastly, a note on description. Big mountains don't always mean old mountains. In the US, the Cascade range is still rising, the Rockies stopped rising 55 million years ago and the Appalachian Mountains finished 260 million years ago (following the Allegheny Orogeny). Stature doesn't imply age. What it looks like today is largely related to the rock of which it's composed, but that is a post for another day.
*I forgive Professor Tolkien, since he was writing before the theory of plate tectonics was formulated
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