The reason behind your knee pain could be your previous weight. do you know that a 1lb of increase in weight could put a 4lbs of weight on knees? May be this could be a cause. But don't worry, you continue walking, the pain will be go to bay when the time comes. Whatever, put motivation and continue...

Can you tell me the exact diet that you are following right now?
Note: This was said by coach and I've followed it, seen a good results.
I think consuming a good amount of carbs, proteins, vitamins and fibre will be the best diet of all time.