Well, this probably won’t help your horse if she’s herd-bound, but to slow my horse’s walk I turn him around towards home… 
It takes us twice as long to get home than it does going out!
Fit him with a set of NB shoes & make sure you follow the protocol exactly how they tell you to on there web page, that will slow him up, it does with thoroughbred race horses .
My guy’s walk is often fast too…I don’t mind a fast walk but it has to be at my request, not his…otherwise he’s the boss which can be dangerous if he’s not thinking and checking back to me. So if it’s too fast or he’s not at the pace I dictate, we stop and back up…stop and back up…ALOT. And I mean 3 steps forward, 10 back…over and over until I see a good improvement. Sometimes it takes us a while to get home, but he tends to want to rush rush back home, and also walks anxiously at times…like “Oh I gotta’ get there…gotta get there or something big and scary will jump out”…like that kind of walk. He’s not spooky at all, just in a rush. If he ignores my half halts, stopping and moving his feet backwards gets him thinking, and I like that vs. a headstrong, fast walk that I did not ask for.
