Watch out when your flatlander feet transition to sloped trails. Years ago, I walked up to Low Gap from Cosby campground and back. It's only five miles round trip, but by the time I got back to the campground, I could hardly walk. It took three days for my feet to get to the point where I could hike again. I originally blamed my boots for the problem, but I think now that it was Achilles Tendinitis. A similar thing happened to me in Canyonlands NP a few years ago, but I paid more attention to the symptoms and stopped sooner. There isn't anyplace in Illinois where you can get the elevation changes that you will find in the east and more so in the west. I walked to the top of Bell Mountain in Missouri a couple of weeks ago, but that was only around 400 feet of elevation gain and 8 miles round trip. I didn't have much time, so I started at the north trailhead. If I had started at the south trailhead, I would have had 700 feet of elevation gain. I had some difficulty walking for the next few days, even though I regularly walk 2 to 4 miles around my home. I haven't been using trekking poles until recently, but I do think that they help me.

Once you get to the Smokies, I would suggest a few short day hikes with some reasonable elevation gain and loss and see how you do. You may not have any trouble, but I would hate to start a long hike and find out that I did have trouble.