I agree that weight is probably a red herring here. Your knee doesn't know how much weight is on the bike, it only knows how much force you are putting into your pedals and your rpm. This is most easily addressed by keeping your gearing in mind at all times. Any time you fomd yourself trying to force the pedals, lower your gear.
As for reducing weight, I wouldn't worry too much about the bike at first. Look at what you are carrying and what you are using. It is much cheaper and more effective to simply leave things at home rather than buying super-light gear.
Next, consider your method of touring. I have this theory that the amount of discomfort experienced on a tour can never be reduced, it can only be moved. Carry a lot of gear and you will live like a king in camp, but may struggle climbing hills. Go with a "stupid-light" load and riding will feel easy, but camping may be less than enjoyable. Or move the discomfort to your wallet with a credit card tour.
I've done a few short tours with "stupid-light" loads of 15 pounds or less. I fashioned my loads after the Ray Way of backpacking. When travelling this light, you must always be aware of your surroundings and what you will be facing in the near future. It is almost impossible to stay warm and dry regardless of the weather with such a light load. Instead, you must pick your camping spots based on local terrain in order to find the spots that will be the warmest and most sheltered. For instance, opt for a grove of trees on the shoulder of hill rather than an exposed hilltop.
Sean