I'm new to BPL, and new to the ultralight philosophy, so I've got a mix of gear and am nowhere near ultralight yet... so I'm still using a Gregory Z-55, because when I was originally looking for a pack, it was the lightest/best/highest-recommended pack I could find from a mainstream manufacturer. That said, carrying 35# in it is pretty easy. I don't think I've ever hiked in it at more than ~38#, but I have no hesitation that it would do 40-45# in relative comfort, especially at the 4-5 mile distances you are talking about.
I'm not adverse to swapping to a different pack, but right now I'm doing most of my hiking with my girlfriend, and so reducing gear weight really comes with 2X cost, since I'm trying to do so for both of us as we go along -- I don't want to be out there with a 15# pack to her #30 pack, and she doesn't have the money for any new gear. So it's likely to be a long, slow process to go ultralight, contingent on finding great clearance specials.
But that said, I like the Z55 well enough that I am not in a hurry to replace it. I give it very high marks for comfort and durability. And for what it's worth, I'm 5'10" and weigh ~160#, and it still works well for me even at 35#+.