Yoga is exercise. Since it requires strength, balance and endurance I would recommend you do it before doing other forms of exercise. You do not want to start doing yoga when your heart is racing and your are breathing hard. Relaxing your body with yoga can also prepare your body for the exercises you do afterwards.
I am in good shape but when I do yoga I would not do a lot of additional exercise afterwards. Yoga with a few body weight exercises for strength afterwards provides me with a good workout.