I weigh about 165-170 lbs. and carry a MountainSmith fanny pack with shoulder straps, total weight of 20-25 lbs., 12-15% of body weight. I find I'm more sensitive to extra body weight than the same load in the pack. I wear this, along with lightweight clothing and the lightest durable boots I can find (Vasque or New Balance goretex), whether the hike is 4 miles or 14. Not hiking as much or as regularly as in the past so I consider it important to maintaining strength to wear it no matter the hike. The pack is stuffed with "basics" which I'm happy to never use but has been sufficient for unplanned overnights in the woods and occasional assistance to damsals in distress (usually blisters).
On the other hand, my backpack weighs in at over 40 lbs., and that starts to get uncomfortable. Admittedly, its the cast iron frying pan, which I insist upon, that blows up the weight ... which helps explain why I've come to enjoy paddle camping so much more!
As an aside, two sentimental items are of as important to safety as just about anything else: 1) my father's WWII antiaircraft compass in canvas case attached to my shoulder strap, and 2) a Vichy German made knife and scabbard salvaged from a soldier who got the worse end of the deal and gifted to me years ago by my Belgian Godmother. Ohhh ... the stories these things can tell around the campfire.