The same way yoga can be an excellent prelude to pranayama, the latter serves as an excellent entry before mindfulness meditation. Hence, once you learn some meditations, you may want to experiment by composing your own sequence, for example:
As your practice advanced and you start seeing benefits of the new state which you achieve with all the mindfulness in your life, you will naturally drift towards extending this time. This is great, however, be aware of going slowly, giving your body and mind enough time to adjust. You may want to extend your yoga practice by 10 minutes every week until you reach the desired lengths of the session. You may want to add body scan after yoga and before pranayama or use one more pranayama just before you start your yoga (Kapalbhati is energizing pranayama which could invigorate you before physical movement).