Try giving more central (or along your sine wave) points on the graph a higher-than-average chance to grow in height and the edges (or farthest points from the sine wave) a hight-than-average chance to shrink.
Instead of hills, you'll end up with craggy mountains in the middle that fade off to slopes and into a sea bed. You can mess with the DS to smooth it if you are looking for more Appalachian mountains than Rockies. Or, and this is my preferred method, use a CA to weather the mountain by eroding away peaks and filling valleys.
You can create ranges of mountains by slapping a bunch of random points in the map rather than just one central one. The math is just a matter of scaling to the distance to the nearest point.