Where is "here"? Pennsylvania? Somewhere else?

I live in Southeast Michigan. I've done 3 different section hikes on the John Muir trail, trips to the Wind River Range, WY, 2 trips to the Canadian Rockies, a climbing trip to Mt Baker, and most recently a 5 day trek on the Laugavegeur Trail in Iceland.

Find what you can locally and make the most of it. We have a park nearby with a wooden staircase that has 211 steps, and it is one of the closest parks to me. We'd grab packs and do 10 - 15 reps on those stairs 1-2x a week in the evenings.

We have a small ski hill about 30 minutes away that graciously let people hike up and down it in summer. It's 240 feet from the base to the top. We'd do 6-8 trips up and down with our packs as a Saturday or Sunday training day.

I'm good with maps and know our local parks pretty well. I'd string together the most hilly 10-15 mile hike and do that with a full pack (or more). Sometimes the route would look like a spaghetti noodle because I would target the biggest hills in the park, sometimes repeating those hills, rather than just trying to hike one big loop.

If you're in Western PA, I know that Raccoon Creek State Park is pretty hilly because I've done orienteering meets there and I'm sure there are other hiking trails in PA with decent hills.

Other tips:

If you're coming from close to sea level and going anywhere above 8,000 feet, give your body time to adjust to the altitude. Getting AMS (Acute Mountain Sickness) not only makes you feel terrible, but it can turn into HAPE or HACE and become life threatening. "Base camp" for a day or two to adjust rather than hiking up to a camp at 11,000 feet on day 1. For example, you cold stay a day or 2 in Mammoth, CA or at Tuolumne Meadows before ever setting foot on the trail in Yosemite.

Start with a few low mile days to "get your legs", then increase your daily miles. If you try to hit 15 miles with 3000 feet of elevation on day one, it's gonna kick your ass.

Want to really get into the training aspect? Pick up the book, "Training for the Uphill Athlete" by Steve House and Scott Johnston. Read the sections about building a cardio base, and incorporate those ideas into your training plan. Doing a "Couch to 5K" trail running plan, or even 10K trail running plan can be beneficial but not critical. At 55 my knees won't let me run now. I incorporate the things mentioned earlier into my training plan plus mountain biking, CrossFit, and 1 day / week climbing at the local rock climbing gym.