Oh yes, it's possible.... the trick is packing light. Also, half of what you carry is "on top": sleeping bag, tent, pad, plus handlebar bag.
We did a 5-week tour a couple years ago, my wife (with the Nashbar bags) carried about 25-30 lbs (fully packed, her bike+gear weighed about 60, I'm guessing her bike alone is about 30 lbs). She didn't have front panniers; this isn't ideal, but she simply refused to carry anymore weight
I had front/rear panniers and carried about 45-50 lbs. Of course, I carried the heavy stuff, and the "shared" gear: tent, stove, food, tools, spare parts -- that's one of the benefits of touring in a small group.
When we went on one of our 4-day tours last summer, we used the same setup, but since we needed less gear, there was room in the panniers for sleeping bags, tent, etc.
If you can't fit 3 weeks of stuff in average-size front/rear panniers, re-pack. Ruthlessly...
Still, when you're on the road you'll need extra space once in a while: new maps, a souvenir, a watermelon for dinner, ... There are two solutions to this: mail stuff home every once in a while. And carry a stuff sack you can strap to the rack. The one I have, I carry my sleeping bag in, but it can expand large enough to carry dinner (including the watermelon) to the next campground.