You're on the right track, but theres a couple of things youve misunderstood here. You're actually passing in the type to describeRun rather than an instance.

func describeRun(runningWorkout: Workout) {
    if let thisShape = runningWorkout as? Run {
        print(thisShape.cadence)
    } else if let ff = runningWorkout as? Workout {
        print("33")
    }
}

let run = Run(cadence: 5, time: 10, distance: 2)
describeRun(runningWorkout: run)

You can only cast specific instances, rather than types.