Do I need a Today's Plan or TrainingPeaks premium account to access planned workouts?

No, the Planned Workout feature comes with five pre-uploaded workouts. However to create more workouts an account is needed. There is a complimentary 30-Day Free Trial on both TrainingPeaks and Today’s Plan.  A Basic account with TrainingPeaks will sync today’s current workout however after the first thirty days a Premium account must be purchased in order to continue using planned workouts via Today’s Plan or a full week of TrainingPeak’s planned workouts.