40 lbs is an odd weight. I carry more like 55 when on long overnight hikes, and less than 30 when on day hikes.
But I have done over 40 miles with a big 60 lb rucksack in one day, and got up and done 36 the next day, and got up again the next day...
If someone were keen and had enough light, say in the summer, it's certainly possible to do 60 in one day.
You'd need preparation, because someone with soft feet would have blisters by mile 30 that were so bad he'd never finish the day and would already be in serious pain.
