In previous years, I've tried to take at least 2 paddling trips with loaded canoe in the weekends prior to my trip.  Each outing lasts about 4-5 hours and really seems to get the paddling arms back in rhythm.  I work out on a pretty regular schedule, so it's mainly just fine tuning the stabilizer muscles involved in paddling.  I've also been known to jog around the neighborhood with my pack and aluminum canoe on my shoulders to get ready.  If you do it, be prepared for odd looks and your neighbors shying away from you.

A month ago I started working out at Koko FitClub,   (You need to Login or Register as recovering from ankle surgery for 4 months left me a lot heavier than I wanted to be.  Customized weightlifting and cardio program that isn't too time intensive.  So far, it's working well.  If you happen to be near one of their franchises, I'd recommend trying them out.  Great concept, minimal hassles, and very easy to stick to the program, which is one of the most important parts of any training.  All the other suggestions sound good to, but that's what I do.