I highly recommend the platypus gravity system. It's what I use. It's simple and light. Soon as I get to camp I fill the 4 liter dirty bag hook it to the filter then hang it from a tree branch or my handle bars. It will filter 4 liters in about 15 to 20 minutes. I set up camp dureing this time. By the time I'm done it's done. If it starts to become slow back flush it with clean water. I usually only do this at home and I add a bit of Clorox. They are available in 2 and 4 liter and about $110 at Amazon. The water bags are durable and can be traveled with full. The dirty water bag opens by a ziplock closure and it does get grit sand dirt in there so after a while it is difficult to seal tightly. Only a problem if your trying to haul it full wich I dont see there ever being a need for that unless your in a desert. (Carrying them both full would give you 8 liters of water ) the filter itself is replaceable last about 1500 to 2000 liters. It's a paper filter cartridge. One issue is winter camping well below freezing if they freeze still wet it will ruin it. I put mine in my sleeping bag or in a bag with chemical hand warmers if it's going to get down in the single digits.
a cheaper alternative is sawyer, it too is a gravity system. Available at walmart for around $30. The water bags it comes with are awful to use there impossible to fill without pouring the water in to it. They just use a regular threaded bottle opening. However the filter itself is decent and will hook up to the standard hydration bladder hose size. If you got a few old camel backs around you can make a good system with that filter. My own experience with sawyer is positive. A few years back I hiked the 1100 mile Florida trail. The platypus would get really slow every other day from the nasty canal and Everglades water. I switched to the sawyer filter. It will hook right up to the platypus bags. The sawyer filter is more porous I know it allowed more crap through but it was much faster and I never got sick. I've never had trouble with the platypus anywhere else. I use it all over the northeast. It makes all water perfectly clear and drinkable. If you are real picky theres an additional charcoal filter you can add. I got one and used it about twice and just didnt see the point. When on the C&O I mostly get my water from the Potomac river. It's fine and its fast moveing. One last thing. I know 4 liters sounds like a lot but by the time you cook dinner fill water bottles for the next day and breakfast before packing up you will use it. The weight savings from the 2 liter isnt worth the extra trip to the river in my opinion. I'm exited for you. I love the canal and Gap I'm on it nearly every weekend