Yes, and those guys train for it. Most of us have desk jobs and would be hard pressed to manage an 80 pound pack.

Long distance hikes are over (semi-)maintained trails. They are usually planned for good weather months. If you get sick or hurt, or the weather does something completely unexepected, you can abort or take a day off. You've worked your way up to it so you are warmed up. And usually your biggest hazards aren't human.

In a bug out, you are starting off cold. You might not have been on the trail in months, it might have been a month since you've even put this pack on. You should expect to have to deal with snow to your knees, or water to the hips- basically, the worst possible weather your area can have. You might need to negotiate fire escapes or maintence tunnels if you are in an urban setting, or take a goat trail down a cliff face. If it's a quake, you'll have to deal with rubble. You might get nailed in a police/military check point and bussed out with your one carry on item that MUST fit in your seat with you (that's the way most extractions from foriegn soil work if they can get the choppers or landing craft in, or chartered flights).

And I freely admit, I learned about bugging out from folks who've mostly done so in the third world, so that is the parameters I plan for. Things like your vehicle being commandeered at gunpoint, and allowed to take your ruck if you are lucky. And if you don't think someone would be carjacked for a nice big, off-road capable vehicle in say, a big quake, I'd say you might have too much faith in humaity.

Hiking the AT or the PT is good, it means you can move a long ways. But I've picked up those packs, tried them on, with the weight in them. They are heavy but not horrifically so, but I'm also a big boy. But I wouldn't want to have to do anything very active wearing one, becuase of the degree of limitation to your mobility and flexibility. And I'd really hate to have three minutes to decided what I need from my pack and have to leave the rest behind becuase it is the only way out; or worse, paint a bullseye on my back with it.

For your requirements, Aligator, it sounds like you are talking about an extension or annex to BOB, maybe something that you could put in the car and then use BOB if you have to ditch the vehicle? Or are you talking a superBOB?