If you attach the backpack to a rack, then OK. A backpack on your back places pressure on points where you do not need anymore pressure than you already have. When I was planning my first long tour I tied weights from a barbell onto the rack and rode around like that. If you have panniers, you can fill two one-gallon milk jugs with water, place one in each pannier on the back, and two more in the front if you want, and get the feel of it that way.
Yah, I agree, a backpack is not such a good idea. I also agree that maintaining a FUN ride is important. When I'm training (right now!) and I have a day where I really resent being on my bike, I take the next day off.
Another good idea is to do a weekend trip to really test out your gear. We're loading up the panniers tomorrow with all of our stuff for our x-country trip this summer and taking it out for a weekend of camping. This way we can experiment with different ways to pack and we can try out some new gear we bought, including a tent and stove. It would suck to head out on tour and discover that there's a flaw with your gear on your first night.
I'm having mixed feelings about the fact that we'll also be testing our rain gear this weekend.