Remember those horses we got for the inquisition? That really important quest in the Hinterlands, that was supposed to make our army move faster etc. Yeah...that is what an army might use to "magically" appear here and there.
It does not matter how many horses you have. The horse would still not be fast enough to get you half way across a continent. Also the assumption is that everyone has a horse. Most foot soldiers would not have a horse. It would require a forced march on the foot soldiers part. The foot soldiers would basically be exhausted one from fighting in Arbor Wilds and secondly from the force march. The army would not make it in time.
The other point is that you can only push a horse for so long at top speed until you have to slow down (watering the horse) or the horse would refuse to continue or drop dead from exhaustion. Even a horse that is breed for endurance would not make it in time. The horse would not be fast enough.
A horse breed for speed like a race horse blows all of endurance in that mile and a quarter.
A well conditioned horse can only maintain a gallop for about 1 mile to 1 and half miles. A horse will be fatigued by the 2 to 2 and half mile mark.
I will give an example unlike the movies Pony Express riders did not run their horses at full gallop all the time. The Pony Express rider alternated between a trot (8-9 miles an hour) and a canter (10-17 miles per horse). The Pony Express rider changed horses every 10-15 miles.
The trot and canter also depended on the type of horse and what was being carried. If the horse was armored it moved slower. Also the horse had to be specifically breed to carry the armor. Those were usually medium weight (1000 to 1200 pounds) to heavy weight horses (1500 to 2000 pounds). The problem is that most of these horses lacked the speed and endurance of light horses (800-1000 pounds)
Light horses could only carry soldiers who were weighing light armor and carrying light weapons.
Unless Master Dennett is providing magical horses there is no way for the army to get there to fight with the Inquisitor.