A Spartan Race will challenge your core strength and stability.
The Russian Twist is very popular core exercise that will build your core strength, as well as improve thoracic (spine) mobility through the activation of your obliques.
To make this movement more challenging hold a Spartan pancake in front of your chest and balance in a v-sit. Rotate through your thoracic spine (move your torso, not just your arms) touching the pancake to the floor as you alternate sides.
Do not rush these, control each repetition.
Note: Russian Twists can be done without a pancake. You should learn to do this without one, in fact. You can: a pretend to hold a pancake or, b opt to put your hands behind your head, aiming to touch your elbow to the ground as you twist to each side. This requires lots of stability.