I've ridden many horses that enjoy running over open country.
Not to be contradictory to others' advice, but if a horse takes off galloping and yours joins in, they're probably not just running because you're pulling against them and they're pulling back. Probably they're excited about running with the other horse, or for some horses they are excited about running alone, and giving them the reins won't make them enjoy it any less or decide to stop.
To clarify: what is she doing exactly? Horses that are enjoying themselves and don't want to slow can try many things and they require different solutions.
When you say "brace," do you mean that when you pull back on the reins it feels like her neck stiffens and she pushes forward into the bit? Or do you feel that there is no response at all?
Or do you feel that there is a less than eager response and it takes a few pulls to slow her? Of course a person must take into consideration that the faster a horse is going, the longer it will take them to slow, just like a car that is going faster (but I assume you are meaning she is not responding by shortening her gait when you ask).
Are you using a "pulley" type rein when you ask her to slow? Meaning, it is much more effective with a strong horse if you are secure through your core in the saddle and from this stable position, put one hand against the horse's neck (you can even hold mane) to stabilize the bit and then use your other hand to pull strongly back toward your core, release and then pull again. This is repeated until you feel the horse respond and begin to slow.
If this is not effective, you can keep the stabilizing hand very secure while pulling the horse's nose actually slightly toward you, which at a gallop or strong canter will cause them to bend, and then switch sides and make them counter bend. This is difficult even for athletic horses, and will cause them to either switch leads or slow, which helps them listen to you and regain a more controllable speed. I have slowed even runaways with the bit in their teeth doing this, after a time.
It is important to have "enough" bit to be able to keep the horse from racing others or rushing off when the footing or timing is not right. In my experience, even if a horse is well trained and responsive in a certain bit such as a snaffle, that does not mean they will be able to go in one in more exciting circumstances. If you look at photos of horses foxhunting and doing other exciting sports with other horses, you will notice many varieties of bits being used.
A Waterford can be helpful, but only if the horse is sensitive to tongue pressure. The first bit I try with stronger horses in the open is a Kimberwicke, and many horses that won't listen when excited in a plain snaffle will respond much better in a Kimberwicke. Last summer when I got together for a beach ride with a few different friends on strong horses, we noticed most of the horses were going in Kimberwickes. For a horse that needs a little more than that, you can go to a myler type curb for added leverage. If a horse does not like a curb chain, then a gag/wonder bit may work.
My friend has been trying to sell a TB that is well trained, beautiful to ride and jump indoors in a snaffle, but can be very strong when out in open country, galloping with other horses. You "can" ride him in a snaffle on the beach, but you're not going to slow him down for quite some time. But in a snaffle an arena by himself, he knows dressage and can transition from a canter to a full stop in a single stride.
My point is that some horses need different bits for different situations.
From 3:00 to 3:20 in this video, if you play it a half speed you can see me putting my left hand down on the horse's neck and using the pulley rein with my right hand to slow this big TB. Both of these horses are off the track and love running.