The reason, more than likely that the other person never had this bolting issue happen, is she had horse in a very controlled environment. Not where there is pasture to take off in. And horses know when someone doesn't know what they are doing with horses, and will take full advantage.
I've had horses that were never any trouble at all for me, or my kids. But let someone else try to work with them? Like the horse had just come in off the open range and was fighting for its life.
Secondly, horses are not dogs. They don't need handwalking except per vet's orders for health issues. She has been in the pasture. She has seen the surroundings, and whether you show her everything or not? She will react as she wants to when she decides to.
And too, if during your "walks" with her, you have been letting her wander, stop when she wants to, go when she wants to, or ignored her behavior? You are setting up the types of problems you are beginning to have with horse.
Humans have to be the leaders, in walking, leading, working around, feeding, or what have you. They aren't your best buddy like a dog, they are horses, and they respond to a strong leader. Any indication that they can boss the human, and they will do so.
And as you have found out, horses think on their own. They can and will decide to do something, and then do it.
I imagine though, horse was giving signs it was contemplating taking off. One ear forwards, one back, a lightness in the body, ignoring you with her eyes, a lift of head...you just missed it, either because you weren't paying attention, or because you don't know what to look for.
Ground work will not do nearly as much as treating this horse like a horse, and putting yourself in the leadership position. If handling in other areas does not change, more ground work will do nothing.
Horses make me a better person.