Kadīd denotes hard or firm ground; murakkal conveys a place repeatedly struck or trampled by feet and hooves. These words anchor the racing image in solid terrain rather than leaving the scene an abstract statement about speed.
Atharna al-ghubār pictures dust rising from repeated hoof action. Hard ground, pounding steps, and dust collectively communicate effort and fatigue among the other mounts. Against this context the extraordinary horse's misaḥḥ becomes all the more impressive.
In the construction, al-murakkal qualifies al-kadīd. The whole phrase gives a concrete setting: hard earth repeatedly beaten under racing hooves.