These bikes can cruise at 65 mph all day long but they are really happiest at around 60 mph.
Try to find away to carry stuff without the backpack, like maybe a pack over the back fender. Backpacks are fine for running errands and commuting, but get tiring and burdensome on long rides. Others may disagree, but my days of wearing a backpack full of crap ended when I left the Marines.
Getting the bike fully broken in helps too. By fully broken in I mean like maybe even couple thousand miles. I would say my bikes feels better than it ever has with about 4k miles on the clock and a few services under its belt.