For me, it was to always bring a big water bottle (or two smaller ones).
I just got back from a hike on one of the hottest days of the year where I ended up walking around 3 hours and several kilometers after running out of water during the hottest part of the day before I could get to a churchyard with an outdoor tap to fill up on. Not fun!
Make sure you’re not pitching in a cold sink. Learned this in Lincoln National Forest, and so glad I had an emergency bivvy in my pack.
What exactly is a cold sink? I’m UK based and haven’t been camping much at all.
https://en.m.wikipedia.org/wiki/Cold-air_pool