My trainer always tells me that if my free walk feels like it's taking so long, then my walk is too slow.
My mare loves to walk at a snail's pace. Lately what I've been doing to get a good and active walk out of her is starting off the ride doing halt/walk transitions. Bascially, I'm looking for her to move from the halt to the walk with the slightest squeeze of my legs. If she doesn't move right away, I'll taptap with the whip. During this, I don't really worry about where her head is. If I'd like a bigger walk, I'll squeeze again and use my whip if that doesn't generate a response. I expect her to hold the pace that I set and if she slow, then my whip is re-inforcement. I school this for like five minutes and after that my horse is moving forward, responsive, and I don't have to use my whip anymore.