Dont get me wrong I like the cracked earth look (I use it on all my Protectorate of Menoth models) but on a larger (3' X 3') terrain piece I dont think it will look quite right.
I am guessing that the pictures above utilizing the crackle medium are not what you are going for, in which case I would look at either the thinned dry wall mud (cheaper though not super durable) or possibly a thin layer of apoxiesculpt (expensive but a bit more durable) with the lines/shapes pressed in using a scuplting knife over a sturdy (non-warping) base. Would recommend working in small sections (floor tiles might be your best bet) I dont know of a quicker way to do this on a large scale.
Edited After thought: Two additional ways to do this, first the long one (that I know works);
Start with a sheet of thin posterboard, use a sharp craft knife and cut the design into the top layer of the posterboard (basically you are just cutting the top layer of the poseterboard). Once your design has been cut use a big paint brush (sponge works better) and wet the top layer. It will curl slightly. Once dry use a wash of your desired finish color. I used this for flooring on a protectorate themed building piece.
Second method (should be shorter but I haven't tried myself) Start with a sheet of cork board (like this ), apply a heavily watered down wash and wait for it to dry. This should cause the lines in the cork board to raise slightly more then the centers and produce the desired sun baked mud effect. Disclaimer I haven't tried this myself for terrain but I have accidentally done this to some cork kitchen heat pads.