I would carry one plastic bag with you. Because the camera is not weather sealed, obviously you aren't going to use it in the rain, right? Then, when it starts to rain, you just put the camera inside a plastic bag, and put the plastic bag with the camera in it to the backpack and keep it there. When the rain stops, you can remove the plastic bag around the camera.
I think the combination of good backpack, plastic bag and some minimal amount of protection within the camera itself will mean you will absolutely have no problems. The rain would have to get through these three layers to damage the camera.
One good choice for such a bag would be a ziploc bag or similar.
Related question for short periods of medium rain, and a camera bag not a backpack: Are typical camera bag materials enough for non weather sealed DSLR in 5 minutes of medium rain?