Advancing from 30 minutes to 3 hours of meditation is not difficult. First you meditate often enough that 30 minutes is easy. Then just increase some more minutes–5 to 10 is perfectly all right. When that becomes easy, then increase a bit more until you manage 3 hours.
You may find that you can only manage to schedule a whole 3 hours once in a week. That is all right. Just let things be easeful and natural and never force yourself. Let your intuition guide you.
Don’t force yourself
As far as discomfort goes, never force your body any more than you should force your mind. It is important to sit upright so the subtle energies flow upward easily, but it is not super crucial. A bit of leaning does no hurt. If you can find a chair that is comfortable, that is good. You do not have to sit so your back is not touching the chair–that is a mythology of fake yoga. Just lean right back.
A comfortable posture
It is not necessary to sit in a cross-legged position, though good if you can. Simple meditation asana is helpful, but not essential. Lotus posture (Padmasana) is for most people much too strenuous and becomes painful in time. Half-lotus is fine, and so is Samanasana, Swastikasana and Sukhasana. Yoga Asanas by Swami Sivananda (Divine Life Society) is very good for learning these postures, and so are the yoga asana books by B. S. K. Iyengar.
Again, no forcing and no making yourself miserable. We are making peace, not war.
- How to Meditate: An Introduction to Soham Yoga Meditation
- Living the Yoga Life: Meditation
- Your Place for Meditation: Tips for Making it the Best