Depending on your weight,
if you have 20lbs to lose, and started running for cardio/endurance.
Being able to run for an hour straight, and losing 20lbs, would essentially put you at your current weight but already carrying a pack!
I'm currently working on just running and increasing my endurance and losing weight. It's slow going though let me tell you!
In the past I would hike with a pack up hills etc... and it was pretty terrible, I was so bored, it was so slow, boring etc... I didn't feel like I was even getting much of a workout and just was bored.
I'd suggest trying to get into running. If you can't, find an active sport/activity you enjoy and start doing it often!
Stairs would probably mimic climbing steep mtns fairly well for us flatlanders...
recently was hiking in CO and man some of those hikes had the quads burning! carrying extra weight/fat certainly doesn't help!
possibly start loading up the backpack and see where you can lose some gear/ a lb here or there...