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!
Bring something to wrap your joints with (the bone kind not the weed kind). I stepped wrong one morning while throwing a ball for my dog and my ankle popped. Suddenly I was keenly aware of just how long the 6 miles back to the car was. Luckily it loosened up enough to walk on after and hour or two, but now I always bring medical elastic tape in addition to my emergency medical kit for when splinting isn’t the best solution.