I have a cleric who wants to ride a war horse (movement speed 60', dash speed 120') with a spell cast on himself. Controlling a mount does not appear to cost an action, you simply decide if you are controlling it or not.
The 5th level cleric casts Spirit Guardians to create a 15' radius, 3d8 kill zone centred on himself. He controls the mount to dash along the front ranks of the orcs staying just out of reach and placing the front two rows into the area of effect.
Riding the 120 feet he forces up to 48 orcs to enter the area of effect (admittedly against their will). If he can hold his concentration he keeps this up for 10 minutes (100 rounds). I'm fairly sure once the orcs start pelting him, his concentration will break but gosh what a first round.
Am I correctly interpreting the way the spell works?