You've got the general idea of it.
Some painted-textures look better than others. The one you were using is quite "flat" looking. Some have some "growth" (corn / wheat / oilseed) on them which give a bit of depth to them and may look better.
Some look better a bit farther away in the distance rather than being seen too close up.
I guess it's a matter of experimenting with what looks good. Maybe put in some more hedges, fences, shrubbery, brush around the boarders of the field and such to make it look a bit more visually appealing too. Add in the odd tractor, silo, barn, etc... too perhaps.
Here's a few examples of some "farmer fields" from my King's Arm Highway Deluxe route
http://www.rryard.com/kings_arm_deluxe.html (the last pic is actually one of the garden vegetable-field "decals" that can be resized and rotated as you like.. probably looks a bit better at more of a distance though:



