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I'm a sucker for military surplus and clunky external frames, so that right there already adds more lbs then I prolly should mess with. Most recently I've done a 15 mile trek with 45 lbs, but the weight isn't what gets to me, it's the lack of padding on my shoulder straps. Rubs into me like a mother fucker so that I gotta take a short break every hour and a half or so.
The rule of thumb I've adopted is to not exceed 1/5th my body weight, of which 45 lbs just barely skirts going over that limit.