My suggestions would be to go from really short time to longer:
drawing 1 – 1 min
drawing 2 – 2 min
drawing 3 – 4 min
until you’ve filled that 20 min space. That way you could work on both gesture (flow) and anatomy.
Gesture drawing is great, but it would probably be wise to use the time to do some a little longer drawings. Best of luck with your practice 🙂