I like PP’s view.
Through time, I became troubled with the whole idea about us deciding pace all the time on trails (not in the arena during schooling though- that’s like a ballet class, ha, ha). I think, on trails, horses should be allowed to move in a pace that is the most efficient to them so that they can remain strong and fresh for a long time. That’s why it is sometimes difficult to find matching trail buddies.
I think, everyone has a natural pace that is the most efficient to him/her and deviating from that pace is tiring and can bring up tightness, soreness, and other issues.
For instance, I am a very fast walker with a long, quick stride. I can outwalk almost anyone and not get tired at all. Make me slowly poke around on a hike with a group and I will be ready to die after about one mile- cranky and sore all over my body (because the pace is in complete opposition to my natural set-up).
Our horse is similar. I never knew, how fast he walked until I started to ride with others. So, naturally, he is in the lead most of the time. He is not even in the lead, he is waaay ahead.
As he is marching forward, I can feel his back swinging and he is loose, relaxed, and soft like a butter all over (and very responsive). It is a wonderful feeling.
If I hold him back, he tightens up, his back stiffens, and his whole attitude changes. He does oblige, but I can tell, the slow pace is tiring to him physically as well as mentally.
So, I would first ask, whether her horse buddies are naturally slower.
Even, if not, I would allow her to lead for the most part and just play little leap frog or stay behind exercises here and there, because, if you are trying to get her “unstuffed,” I think, it is best accomplished with low-pressure, fun rides, when a horse gets a lot of free rein.
Of course, I assume, your mare is generally obedient and listening to you, when in lead.