Nothing more than a lot of walking and running walk. When she starts pacing I slow her down and get her back into the right gear. I also have a lot of hills here, more hill than flat.
I did the same thing with my old mare. She was race trained and wanted to pace at speed. I wouldn't let her do it much because of her age and she kind of fell into the run walk and rack as a horsey protest. When she did it I encouraged it. Made her hold the gait. Eventually she would just fall into it and go on a loose rein.
Yup, that's pretty much how to do it!
Our second Walker was a three year old Sir Winston gelding that paced like a camel. After doing all the wrong things (heavy shoes, messing with foot angles, 9" shank bit, etc.) I got some "education" and trashed all the things the "experts" were telling me.
Our program was to put the old boy in snaffle, take up the contact, and then start at the dog walk. Then ask for just a little bit more with the leg. As soon as he started to pace then back off to a correct form and keep him there for a few days. Then ask a again for a small amount of increased speed with the leg. As soon as the pace appeared, back off to that correct form. Sessions were seldom more than 45 min. and we worked five days out of seven.
This "stair step" worked very well. It took almost 90 days of work to get a decent running walk out of him but we did it. This program also has the advantage of really "legging up" a horse. Many horses don't gait well because they are weak and unfit.
One thing I didn't do then, but would do now, is add the canter to program. This is a very good exercise to build wind and it "breaks up" the pace by requiring the horse to use himself in a different fashion. We're talking about once or twice around the ring in each direction, here. Don't get too enthusiastic.
This is really a slow process but it pays huge dividends.