I'm just getting into the backpacking thing myself, so no advice there. But, I have been going on day hikes for decades, Whitney being the longest at 22 miles (though Mt Shasta was probably the toughest at only 14 miles). Personally, I've started riding a bike for cardio during the week with longer rides and weather permitting, hikes on the weekend. Riding a bike even more than hiking has given me an appreciation for how much difference a few less pounds can make. Lol. While riding a bike can't duplicate the potential physiological response to elevation, it does a good job of preparing your cardio system for the hike. I've never had a problem with AMS (knock on wood) so the cardio is all I care about.