>>2826157You just need to work on a decent cardio base during the week. Get a heart rate monitor and stay in (very roughly) zone 2 every morning, doesn't have to be perfect. You can run, bike, treadmill, whatever. I'm saving up for a treadmill that can do 15% inclines to swap my cheap manual indoor bike out for, but tbdesu the bike has done a lot for me when I hike on the weekend. Running would be even better.
My other biggest tips are: aim to eat 300 calories of sugar an hour while hiking uphill (I use shotbloks because that's what I grew up doing; you'll probably be burning a lot more than this) and drink water with electrolytes like nuun added. Stop every hour for 5-10 minutes. And go full ultralight gram weenie on your gear, I do not for an instant regret it. Of course if your goal is training don't worry about this as much, but on big days I love my super light pack.