Ironically my efforts to improve my horse's walk is to consciously do as little as possible to interfere with it. I had always scored in the 7's with my horse's working walk and free walk. (we're just doing training level so no collected walk) Last summer, in an attempt to improve my seat, I began using a new trainer who came highly recommended. She had me doing all sorts of adjustments with my seat to enhance his walk and was continually telling me to slow him down. Next dressage test, for the first time ever, we got busted for a lateral walk and scored a 4. Ugh!! Ever since then, my method of improving the walk has been to try to match my seat/hips/lower back to exactly what my horse was doing. Instead of try to manipulate it, I now try to match what is offered. I'm firmly in the school of thought that you cannot change a horse's walk without making it un-pure. All you can do is match it and let the horse shine through.
You just have to see your distance...you don't have to like it.