I did my first loaded tour a few weekends ago and I was wiped. I went about 35 miles (maybe more -- my computer was malfunctioning and packed up the next day) in 5 hours over hilly terrain. Returned the next day on a more direct route of about 30 miles in 4 hours. This included descending the very long climb that started my ride the previous day. I felt I was going really slow for much of it. I stopped to eat blackberries from the side of the road though, and took pics when I found a particularly nice vista, trying to make the most of my slow pace!
I was surprised how shattered I felt when I arrived in camp. I wasn't as sore after the return journey however. I don't think I would have doubled my distance that day over the previous day, however!
What I found was worst was how sore/numb my bum was! My muscles could have gone further but my bum protested. This has led to my decision to try a Brooks saddle.