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.
The heart of the path is SO simple. No need for long explanations. Give up clinging to love and hate, just rest with things as they are. That is all I do in my own practice. Do not try to become anything. Do not make yourself into anything. Do not be a meditator. Do not become enlightened. When you sit, let it be. When you walk, let it be. Grasp at nothing. Resist nothing. Of course, there are dozens of meditation techniques to develop samadhi and many kinds of vipassana. But it all comes back to this - just let it all be. Step over here where it is cool, out of the battle. - Ajahn Chah