an exercise bike is ok, but putting some weight in your panniers and doing hills regularly will prepare your legs in a way that you cant do on an unloaded bike or an exercise bike.
Doing gradually longer rides on your unloaded bike, and doing hills with some weight is a pretty simple way to get your leg and knee muscles stronger and specifically with the hill training--gets your head ready for working hard going up hills with a bike that weighs maybe 30 or 40lbs more than it does unloaded.

btw, camping planned? ie carrying tent, stove etc etc?