Yes I agree, you CAN! It just isn't easy. I'm dealing with a Pony walk that is short and SLOW and gets a lateral tendency when he feels nervous, so I'm doing a lot of leg yield in the walk, and not riding with too much contact at the walk. And it is getting better - the lateral tendency is pretty much gone, and the energy is getting better, and the strides are getting longer. When I ask for more "go", that use to mean jig, or kick, or NOTHING. Now he goes- at least for a few steps

Our warm up every day is leg yield on the diagonal, then turn on the forehand... I know we'll never have an 8 walk, but it is going from a 5 to a 6 to 7...
So different from my mare who has a great walk...
I think a lot of walk issues are tension related. And some horses just have too BIG a walk, so collecting it is stressful to the horse (aka tension)...
Congrats - 8 is a big increase!