You may meditate sitting in a chair. Keep your spine erect-no leaning on the back of the chair- and your feet flat on the ground.
Thirty minutes is not bad for an "ouch" response. Even on long retreats one generally sits for 40 minutes at a time, then walk, then sit...
and as an Artists Model, i seldom hold a pose for more than 20.great practice- notice the pain, don't suffer the pain.
However in my fifteen years of meditation training no teacher has ever said you must sit in one, prescribed position for any specific lenghth of time.
In Zen one is instructed to sit in a specific way, basically the one described above & that is the position i use but i have not studied Zen meditation so i don't know how they feel about changing it.