Sitting hours hours on end and meditating is not possible for most people except for the exceptional few who can enter into deep states of concentration. Take it slow and experiment with different postures. Try using a bench or sitting in a chair. Take it slow and try sitting for just 20 minutes if you can. Its important to remember though that whatever posture you take, if you sit long enough pain will eventually arise. This is the nature of the body. The key is to not let the mind be pained along with the body.Gunaratana is not saying to use pain as your main focus. Pain is to be treated as a secondary focus. Watch the pain as it arises, peaks and then fades away. If the pain is too severe you may have to move but dont move at the first instance of pain. Push your self a little. You will be amazed at what you can sit through with patience and not automatically react. If you have medical issues sitting may not be an option. Keep in mind many monks have become enlightened on the walking path.
This is our foundation: to have sati, recollection, and sampajañña, self-awareness, whether standing, walking, sitting, or reclining. Whatever arises, just leave it be, don't cling to it. Be it like or dislike, happiness or suffering, doubt or certainty... Don't try to label everything, just know it. See that all the things that arise in the mind are simply sensations. They are transient. They arise, exist and cease. That's all there is to them, they have no self or being, they are neither ''us'' nor ''them.'' They are not worthy of clinging to, any of them. - Ajahn Chah