Hubby did about 43 miles on the Adirondack trails with some buddies a few years ago and didn't train at all, but we were both fairly frequent gym visitors at that time.
For our PCT hike, we'd been going to the gym regularly, (I was working at my daughter's gym at the time, BC--Before Corona) so we put on our packs and hiked some of the trails around here, usually about 4-7 miles every day for the two weeks before we left, with a one-day break before hitting the trail. We spent some extra treadmill time for about a month before that, sometimes wearing a small pack, too. We wore the clothes and footwear we planned to use on the trail, and mostly had all our gear in our packs. We ended up needing to replace some food items because we used them during our conditioning. We chose to use the water bladders we adapted to have a quick-disconnect for our Sawyer mini filters because of the convenience, and my side pockets left my water bottles out of reach without taking off my pack. We always try out new foods and gear before we do any trips (we'd done several overnight backpacking trips previously). One time we decided to try a new Mountain House meal on an overnighter and didn't like it, but had brought extra just in case. Good thing!
One thing that I saw on a YouTube video was the use of a "pee rag", clipped to the side of a pack. It's a 1/4 of a cotton bandana that is only used for pee, and then cycled out with others as they UV-sterilize in the sun (there wasn't much of that!) and rinse in streams. I saw these clipped to packs of other women I saw, too. We also practice good outdoor niceness by packing out our used TP for BMs, or burning the paper when we camped where we could have fires. I know that part isn't actually fitness-related but if you're backpacking, it's an important thing to consider!
We estimated about 10 miles a day, and our first day (which started at straight-up noon) we got 6.5 miles, then 12 miles the next day....we actually planned our nearo day for a day later, but I pushed us to get to Elk Lake Resort the last night to rest longer. When we got back on the trail to do the second half, I started thinking and it went downhill from there. It was the most challenging part of the hike because of greater elevation changes, drier stretches, more rugged terrain, and needing to actually use Guthooks to navigate through snow where the trail was hard to find. I realized my insoles were the reason my feet were always hurting when we stopped at Elk Lake. The ONLY reason to continue was to be able to brag that we'd done it, and that wasn't a good reason for me. I wanted us to do it for US. As it turned out, most of our friends were pretty impressed with doing 50 miles anyway, so that was a good call!
Thanks for the input; it sounds like there are a couple tweaks we can make for the next trip, and I liked reading about your experiences, too! If you want to join us, we'll be going in late July or early August, when some of the through-hikers will be starting to do that section, too. We're going to go a little slower so we can enjoy the scenery more this time, though, maybe 8 miles a day.