Sure, you should warm up first before attempting yogasanas and pranayamas and this warm up can be done even by running or jogging for 20 minutes and it is enough. After running, you can do some stretching exercises - twisting, forwarding - full body extension /flexion.
I think then your flexibility will increase and then, you will be able to do yogasanas easily. However, I may also mention that flexibiilty for yogasanas shall take time to increase and for that you should also add sukshama vyayama (designed by Swami Dhirendra Brahmachaari) in your exercise schedule.
I hope that the above clue helps you. Remember to do relaxation after warm up and before yogasanas session and then between yogasanas and pranayamas session and then at end, you can do full body progressive relaxation / Deep relaxation technique (DRT).
