Agree with Blueberry. 40 miles unloaded is not really the same as 70-80 loaded unless the 40 miles is hilly and the tour is flat!
When we were training for our 9-day self-supported tour in very hilly country (Finger Lakes and Adirondacks), we made sure we had ridden several back-to-back long days (each weekend day for a few weeks ahead of the tour). We also took one two-day shakedown tour, which I highly recommend. We loaded up our panniers with a similar load as we'd have on the tour and on a Saturday, rode similar mileage on a relatively hilly route to a town about 60 miles away, spent the night in a hotel, and rode home on Sunday.
To me, the most important things were getting my azz ready to ride in the saddle day after day, making sure I had shorts that worked well for me, and hillwork. I always spin, so didn't worry about my knees.
Good luck! Touring is such fun.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow