No one "needs" a power meter. I've done a couple of comparable tours to what you are planning, and I am not sure anyone (except maybe pro racers) had even heard of power meters back then. Sure, I guess they can help you become a stronger rider, but if you just want to increase endurance to enjoy your tour, the best way is just to get out and ride! Gradually increase your mileage and add back-to-back days. You have to toughen up your saddle area as well as your quads and other cycling muscles (core, etc.) If the tour will be hilly, you need to do hill work. It's really quite simple and no special tools are required unless you enjoy such gadgets.
Good luck and have 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