As said many times here, and from what I have seen taking friends out from the Midwest,
Best way to prepare for hiking many steep miles with a heavy pack, is to hike many steep miles with a heavy pack.
Hardest thing is to comprehend the terrain you will be in. Hard to replicate in the Midwest. Try hiking road ditches to replicate side hills, and find the steepest chunk of terrain you can around home and walk up and down for as many hours as you have available, and zig zag to replicate elevation and side hills. Consciously lift your feet to replicate dead fall and navigating through brush.
You may look and feel like an idiot, but it's the only way to replicate the terrain you will be in without actually being there.
You can try to isolate cardio on a road bike, and isolate leg muscles with different exercises, but In my opinion it's just not the same as 10 mile days in the mountains.
Do that mostly with day pack weight, but then throw on 100lbs every now and then to replicate a rear quarter and an elk rack. That's when you get into spirit world, dehydrated at 2am with the last load to the truck lol
God speed, you will be happy you prepared when your hucking up a ridge at a bugling bull, rather than huffing and puffing at the bottom.