For true backcountry travel of up to that length, I use a 50-60L backpack, with food taking up about half of the space. I use a cheap "HighGear" backpack for that application, which has flaws but gets the job done. If you will be around towns or can make short detours to where food is available, I recommend buying food every couple days, so that you can travel faster and carry a smaller pack (30-45L). I use a GoLite Jam for that application, which has served me well for six or so years, and is still going strong and very light (1lb 5oz)