I'm gonna stress again this potential heroic endeavor is still a "maybe" at this point in time, tho I have begun actively preparing for it. 40 arid miles between town seems pretty usual in the West, an extra gallon of water or maybe two of on hand would be a sensible precaution at times. I'll have to browse CGOAB to see what others have done.

Actually, on the flat or slight grades, a 90lb bike isn't all that tough, momentum being what it is. It's gravity that make it tough, when climbing. Point of interest, IIRC the longest I have ever ridden in one day, a 110 mile training ride, was on my bike weighing ~75 pounds, loaded up as if to travel one month before my 2016 NY trip. Pretty warm day too IIRC.

Didn't plan it that way, I left the house around 10am on an errand, maybe a 20 mile round trip, but it was such a nice day I kept on going, rolled back up my driveway around 11pm that night. 13 hours, I never have been a fast rider.

Doesn't your lower back hurt (more)? Don't your knees hurt (more)? If the answer to both those questions is no, fine!

I am fairly leggy and 5'11" I never used anything other than 175 until a few years ago when I tried 170's. My battered knees thanked me. It's crazy to think that 5 mm can make such a profound difference.

well one of you is making this stuff up, so I vote for an arm wrestling contest.

I'm 5'10, or at least was 5'10 30 years ago....

Oh, my body gets battered with riding. Knees, back, whatever.

Yet, in reality, the old bike was 172.5. Now, I think every bike has either 175 or 180 cranks.

And, the knees and back feel better now than they have in the last 20 years.

Of course, there have been other changes, and I'm putting a lot more miles on the bike which I think do help. The body is somewhat addicted to the bike.

Ibuprofen might help..

Longer cranks? No, shorter cranks. Not going to bother with explaining why they work well - at least for some. Very odd that most all who want to try something different always head for bigger is better. Until you have tried a variety of lengths you just don't know. Very few are ever going to do such an experiment. IMO 170 is already way too long for someone 5'6". Also IMO so few have tried a range of lengths that categorical assertions have no weight. Standard 170mm or 6-3/4" cranks are just an historical accident. If you are curious and can afford to play I would suggest an experiment at 150mm. Experiments are good. Curiousity is good.

I am 5'8" and have used Rohlof14 and 180s on 2 tours, 4,200 at 61 and 3,900 at 64 yo. My knees gave me not a single squeak. Going up and down stairs is far more likely to feel something. What did ache was was left achilles the second week and cramps most days. Lurching on and off my 120 lb bike to take photos took getting used to.
When I started using IGH bikes, I really felt the difference with 165 mm to 180.
My SA 3 spd 1973 CCM 1Pc has 167.5. It does feel rather lame and shifting to 2nd is tough.
My 1974 23" Raleigh had 165s, no wonder I never really liked it and barely went for 52 mile rides.

Bike fit will also make a signifcant diference - like seat height, saddle fore/after adjustment and handlebar reach/height. You have to adjust it to either a shorter or a longer crank, each time you swap the cranks, even if just for trying what you like best. Otherwise the diferent cranks might produce irrelavant or misleading results. Aditionally, some people just do feel better spining (shorter cranks) while others wouldn't trade lower cadence for anything else.

For your heigh, makes sense! For mine - 5'5 - doesn't. I have tried 165, 170 and 175mm, and both my lower back and Strava love the 165mm ;-) Oh, and all other things being equal, spining lower gears (with shorter cranks) always saves your knees compared to smashing higher gears (with longer cranks). 180mm might be a little to much, imo.

I agree completely with all of these important details.
As with all bike fit stuff, as you say, small changes make a big diff and clearly, crank length changes interact with all the other variables you mention.
Internet answers can be easy and fast, but as experienced riders know, its the small little positional changes that make all the difference, and get shown up in long duration, relatively hard effort stuff like racing and or touring where we are not going fast, but working a lot harder than unloaded riders.
The fact that at my height, nearly 5ft 11, I can use 175s on a couple of bikes, one which I have spent months on touring, means the seat position etc is okay for my knee angle etc so that I can ride a heavy touring bike in mountains and not have knee issues--yet I am not a masher rider by any means.