For a trip that you describe, I would actually recommend using both a rear rack system and a front rack system. I have not used a front rack system, but I understand that it changes the handling of the bike. Having basically four panniers, two on front and two on back, will allow you to distribute the weight around so that you are left/right balanced and neither of the tires is taking an excess. Then you can redistribute to get the handling that you want.
The people who run The Path Less Pedalled went across the United States with just such a setup before they bought Bromptons and changed everything.
For rear panniers, maybe one of the Topeak units that consists of a trunk and two side panniers. Alternately, an Ortlieb set of panniers, leaving the rack proper for you to rope on gear that doesn't fit nicely in the bags (I'm thinking, say, a sleeping bag or hiking poles). I'm not sure what to use for the front, though. Your bike shop should be able to help out with that.
I'll agree with all of the other comments in that a backpack will both put a lot of painful strain on your back and throw off your balance.
Finally, keep your tire pressure within spec and do be careful on the forest trails. If you are familiar with off-road biking, understand that your agility is going to be far less than you are accustomed to.
Good luck! Maybe also start a weblog and post your progress frequently. ^__^