Standing practice will bring up many discomforts if you
train regularly. The question is: Is it really pain you’re experiencing? Pain is a sign that there
is something wrong. So first check that your alignments are correct. If what
you’re actually experiencing is discomfort, then it is due to the posture targeting
bound tension in your body. If you don’t practice standing, you may not release
those tensions. However, you should not force yourself to stand for any certain
period of time. So you have to feel your way through it. My advice is: If it is
truly pain, then you should sit. If it’s actually discomfort you’re
experiencing, then stand and, once the discomfort becomes obvious, remain
standing for a couple more minutes (if you can), and then sit down for the
rest of your practice. This process will slowly open up the body, release
tension and allow you to stand for longer periods without discomfort. If you
just sit and wait, you may never be able to stand for 20 minutes without
discomfort.
The answer to your second question is yes–just standing and
breathing with attention on refining your alignments.