"In fact, the only exercise that will really get you in shape for carrying a pack at high altitudes is carrying a pack at high altitudes."

That's why IMO longer trips can be so great --- after the first week or two you've literally walked yourself into a better state of fitness and can enjoy the benefits of that going forward.

Where a really long trip isn't practical (or perhaps desirable), certainly reducing pack weight helps. About all I do otherwise is go for long-ish walks in the area I live, to include sometimes some areas that are somewhat steeply up-and-down.

Another trick is to backpack with people in even worse shape (and/or with heavier packs) than you, so you feel strong by comparison!