Let's talk about what options you may want in a bag. For longer walks it may be of benefit to have bottom compression straps and an adjustable shoulder harness. These distribute the weight more evenly on your body. You may also want to consider a bag with some type of rain cover to keep your items dry. Any bag under 35 litres may not have enough space to hold all your gear.
Below are photos of different disasters and emergencies. Click one to see what size bag and items you may want to pack.