I've heard that treated down can start to clump and you end up in the same boat losing loft. In a real wet environment like that, I'd go down in a water resistant bivy.
I started out with a Remington 11 20ga and my dad had the Fox Sterlingworth 12ga when we rarely went out duck hunting. I inherited it not long ago and got some low pressure rounds to continue to enjoy it. Elegant gun.
My wife ordered some clothing from Paka and the usually give a free pair of socks with a purchase so she gave them to me.
Now my favorite socks. I wear them everywhere and there is no sign of wear. Cushioned just right and no stink on a longer trip.
HIIT workouts can definitely help. They will help you get more efficient at recovery after effort.
Spending a day or two at elevation before you start out will help acclimatize.
Staying hydrated is always helpful. Not everyone is affected by altitude the same.
If you are looking to stock up emergency/survivalist food or just really like the taste of your own meals at camp, it's worth looking into if you have the money
Just for lightweight food for the occasional trip, you are better off buying pre-packaged stuff or dehydrating food (lot cheaper and...
I go this route. I'm a fan of layering to dial in the warmth you need.
Same with pads. I usually take a thermarest zlite for the extra r value and to protect the pad from puncture.