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.
“What should be done for his disciples out of compassion by a teacher who seeks their welfare and has compassion for them, that I have done for you, bhikkhus. There are these roots of trees, these empty huts. Meditate, bhikkhus, do not delay or else you will regret it later. This is our instruction to you.” - MN 19