>>3385087 liters/ 2 gallons (sorry for using metric all the time, I have to convert it and then it's just easier to keep thinking in the unit I'm used to) weight 7kg. That's almost half the weight you will be carrying already - any more than 20 to maybe 25kg is just not reasonable.
You should instead try to plan your route in a way that allows you to regularly refill your water.
Route planning is in fact pretty important for a longer hike. Try to plan alternatives. What if you overestimate yourself? Plan in some shortcuts or a way to end the hike early. What if you underestimate yourself? Think about possible extensions. Maybe a day trip to some lake nearby. How about weather? If the weather gets bad, will you be able to continue? Or will you have to cross an area that becomes impassable when there's too much rain?
Plan in rest days. If from experience you walk, say 20km without problem, plan for 15km day trips. Additional weight will reduce that even more. Sure you can force yourself through more than that, but you won't be able to do it for 7 days straight.
As for clothes, don't even consider bringing any cotton. Wool and fleece for insulation, softshell or hardshell (goretex, etc) to protect you from weather and synthetic underwear to wick moisture away from your body.
Of course you can wear jeans and a cotton shirt for a day hike in good weather, but you can't plan for good weather if you want to go on a 300 mile trip.
Make sure you have the proper footwear for whatever environment you're going through.
If you already bring an extra pair of socks, bring another extra pair of socks.
Always, and I mean ALWAYS pack your toilet paper in separate plastic bags. In fact, pack everything in separate plastic bags. Especially food and toilet paper. The toilet paper so water doesn't get in, the food so it doesn't get out and smear all over the rest of your equipment.
Have some other valuable things? Paper? Notebook? Headlight with batteries? Pack it in plastic bags.
