I am training a walking horse right now and here is what i have learned about the breed and their gaits.
the desired gait is a 4-beat with a headshake. Although most walking horses are naturally able to gait this way, not all of them use their 4-beat gait 100% of the time. It isn't an easy gait for them, so in the pasture they slip out and pacem or trot. Pacing is 2 beat, very rough and not very comfortable to ride. It is when the front and back legs of the horse on each side work in unison.
When they say your horse tends to pace she slips out of gait, her ride becomes rough. If she were racking, she would still be in a 4-beat motion but with no headshake, in the rack it is still smooth but they are able to skip the headahake and have more speed. There are actually shows specifiocally for racking horses as it is to some people a desirable gait.
So when they say she will pace on you if you go too fast, she doesn not have the muscle build up to go in a fast speed at a 4-beat walk with a head shake (she is not racking) the fact that she paces if you let her go too fast isn't a bad thing...you could fix it by doing a lot of trail rides and arena work and slowly working your way up to a higher speed as her muscles develop.
My girl does the same thing, she is 3 (and i am traiing her for show so working on these things soecifically right now) and when she gets going too fast for her muscles she switches gait to something she can keep her balance at in the speed she is going. She step-paces, keeps her balance and maintains speed rather than form.
So it is a fixable thing, i would go out ride her, see how she feels, some horses (like mine) do a step-pace which is a very smooth pace, and if it is to be a trail horse, a step pace is workable in the trails as well as you aren't bouncing around.