I used to be a llama guide here in Mt. We would seldom get more than 5-6 miles of aggressive hiking out of them per day. Anything further than that and you might have a 300 + lb wool rug laying in the trail. Once they lay down, it can be hard to get them up again. As for as weight, we would load them with between 40-80 lbs dependent on their age and sizes. They are interesting creatures, not the most friendly pets though.
The spitting is way over blown. They will spit at each other, but the only time I ever had one spit at me was when they were sick and we were trying to treat.