I wouldn’t be particularly concerned about getting a lot of elevation-change training if you’ll be doing most of your hiking on flat ground. If you do decide to do a hike with hills, I wouldn’t jump onto the biggest hills I could find for the first trip though. I’d do some training trips into the foothills first - and I would jump on that treadmill to train for the foothills.

For example, on that nearby trail I mentioned, there’s never more that a hundred feet of elevation gain/loss at a time (about 6 such gain-and-loss hills in a day.) However, if I know I’ll be heading to Kentucky, I’ll do some training hikes a bit southeast of us, where there are some real hills to climb. I’ll schedule an easy hike the first Kentucky day, and then adjust my trip plan as necessary.