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!
I’ve been listening to the Out Alive podcast for a little while and it’s amazing to me how many of those scenarios could have been resolved much quicker had they been able to SOS.