The marching speed of the ground troops compared to the aerial ones was vastly different.
The ground troops had to go over terrain, go around large boulders, pass through narrow valley passes, walk over plains, but the aerial troops just had to fly.
To synchronise the speed of flight with the movement speed of the ground troops, the dragon's often circled around the same spot as if they were preying eagles, so that the ground troops could cover enough distance for the march to continue.
After the vampire troops diverged towards their specific attacking positions, it only took them 1-1.5 hours worth of march to exit the proper land domain and enter the desert sands.
Fine black sand mixed with normal sand covered the entire expanse as far as one's eye could see, and a troop had to go knee deep into the sand just to get enough footing to take another step reducing their marching speed considerably.
