Reading height from a model would probably require reading in all of the vertices, which means you'd have to create your own vertex reader.
I would do a google on heightmaps, or check the tutorial at www.riemers.net. A heightmap can simple take a bitmap file (greyscale is easiest) and use that to create a terrain. With a heightmap, XNA lets you read the color information on the bitmap file and you can use that color value as an elevation or height. Again, the tutorials show this.