mirage,
One aspect of sitting in padmasana for longer stretches of time is to make certain that you do not sit directly with the body touching the floor. If you do this, within on average fifteen to twenty minutes, you will start to feel certain parts of the body becoming numb because the direct contact will cut off the blood circulation from flowing efficiently through one’s system. To solve this problem, one should sit on a cushion or something that can offer a protective padding. This will help the blood to circulate without obstruction, as well as assist prana to flow through one’s system.
Besides this, the more you practice sitting in padmasana is the most your body will gradually adapt to the posture. The muscles of the pelvic region as well as the heels will start to become more flexible. Since you are practicing already for 20 minutes in padmasana, start practicing in the posture for 30 minutes. Once you have practiced for enough time and you can sit in the posture more or less effortlessly without too much discomfort, increase the time to 40 minutes and do the same. Like this - gradually, you will be able to sit for hours upon hours in padmasana without too many problems.
Another factor to consider is strengthening of the spinal cord. This will happen naturally as you continue sitting in an erect posture. Otherwise, what is also of tremendous help is to practice asanas which involve bending of the spinal cord, such as dhanurasana, chakrasana, or matsyendrasana.