Here is an obvious one: you will have to be paddling on fresh water if you are filtering. I prefer filtering if the water source is suitable, but lots of places, it's not. This may be because it is brackish, too silty, or polluted with industrial or agricultural run off.
Silt can clog the filter system, so you may want to bring whatever it takes to unclog. I have been using a Katadyn PUR, so I bring a replacement filter. In the future, I'll switch to a gravity feed system--they are much easier--and don't know how you unclog those.
If you are relying on water collected on route, bring back up against failure of your water system. Easiest and most packable are iodine tabs. Doesn't sound like you will be cooking, but if you have a stove, boiling can work, but costs you some in extra fuel.
People do pack 5-days water on trips. Water bladders are probably more packable than hard containers.
Other than that, a 5-day trip is like a 3-day trip. You just need more consumables. And you might be happy to have what it takes to do a batch of laundry, although that requires being stopped long enough for clothes to dry, or at least drip mostly dry. With modern outdoors wear, and depending on temps, you can put stuff on that is damp and it dries quickly.