Your backpack is essentially you're long term Bug out bag; there's really no difference. It's purpose is not to carry everything you need, but rather to carry the means TO GET what you need. In the early days of any disaster that puts you out of your home, you'll need to move fast and efficiently. Carrying food and water may be necessary in the early days but there's no way you can carry enough to sustain you for more than a week or two. If your plan is to see theapocolypse through to the end, then you're gonna need some things. A couple of essential items include the means to start a fire (preferably several different methods) and the means to purify water (again, preferably several different methods). A good first aid kit is also a must, including a good diarrhea and indigestion medicine. You have no idea what you may have to resort to when it comes to eating and the last thing you need is digestion issues. As well as being, let's say,"inconvenient", diarrhea and/or vomiting can cause weakness, severe dehydration, and eventually death. These are just a few of the things we suggest from the literally thousands of survival items available for your pack. From here, the items you'll need will vary greatly depending on things like geography, weather, and your long term goals. Keep in mind, however, everything you carry in your pack has weight and it adds up very quickly. An over sized or over loaded backpack will only slow you down and just might get you killed. Choose the appropriate sized pack for your body size, capabilities, and needs; otherwise, the only person that will benefit from your supplies will be the first "prepper" that comes upon your corpse.
If you're prepared for zombies, You're prepared for anything!
Colorado Zombie Outpost