Let me preface this answer with a tale from my part of the country. A few years back, there was an experienced trail runner who went for an afternoon jog in November on the trails around Sedona, Arizona. It was a sunny, 70 degree day. He got turned around on his return trip and was quite lost by the time the sun went down. Nighttime temps dipped into the 20s. Dressed in only shorts, shoes, and two upper body layers, he succumbed to hypothermia by sunrise.
So, the first thing to bring on the trail is the mindset that Murphy's Law operates in the wilds too. When I go for a short run with my dogs, even if it is close to my house (we live in the woods), I have a "reductionist survival kit" that accompanies me.
It's all contained in a small Kelty fannypack and consists of survival items I regularly practice with.
Here's my usual gear:
1 spark rod firestarter
Cottonballs with Vaseline
Swiss Army knife
1 quart Nalgene water bottle
1 large orange trash bag
Atwater Carey mini First-Aid Kit
With this simple kit, I have the means of taking care of shelter (trash bag), signaling (mirror, flashlight, and cellphone), water, firemaking (spark rod
cottonballs), medical, and foodenergy (Clif bar and knifefor Rambo style hunting, just kidding).
Regardless of your activities in the wilderness, you are always going to have to prepare for the survival priorities of shelter, water, fire, signaling, and, potentially, first-aid. For longer hikes or trips you will need more robust packs.
If you don't want to assemble your own kit then obtain a quality commercial one like Doug Ritter's Survival Kit by AMK. He put a lot of thought into this lightweight, affordable kit and it will cover the basics should an unexpected night in the wilds befall you. Just make sure you practice with anything you are going to stake your life on and remember to always leave a travel plan with someone indicating the location of your run.