I can contribute a couple of points. I work with two brothers. They pedaled across the US together and spent $.80 on food and about $5.00 on coffee for the whole trip. After that, they rode from Alaska to Tierra del Fuego with similar expenditures. And took the Continental Divide trail to get off the roads and make it more interesting. They did stop to work a couple times in South America on that trip. So, uh, not an issue, but you have to be flexible in your habits and expectations at that level.

I have a bad back, too - spinal stenosis and two or three other degenerative issues. You are far better off bent over a traditional road bike than in a more upright position. I can ride with my bars below my saddle a lot better than I can walk. The reason is that when you are upright, your back is in column, and all the jarring travels right up your spine like a pile driver. When you are bent over, your butt goes up and down but all that does is flex your spine a little, which is a good thing. It needs the exercise.

The other good thing about being bent over is that it opens up the sides and back of your vertebrae which relieves the pressure on the discs and nerves. There are very good reasons for the standard road bike position, and wind resistance is not all that great a part of it, really. The modern MTB position is about the same, but MTB bars have way fewer hand positions than drop bars and are tiring on long rides.

In other countries, and out of the way places, 26" parts are much more available than 700c. South of the US border outside of large cities, no one knows what 700c is. And 26" wheels are stronger because they are smaller and have better spoke angles. My wife has a randonee bike with 26" wheels, MTB gearing, road bike bars and shifters, and full touring eyelets and braze-ons. It's pretty neat.

All that said, you may feel that I have it all wrong because it's uncomfortable for you now. But I can pretty much guarantee you that you'll feel very different about it in a few thousand miles. Which you should start accumulating now. Work up to riding 150 miles/week to begin with. Go to the gym. Lift some weights. Squats, seated rows, benches, dumbbell presses, lat pulls, crunches, leg lifts, lunges, all that stuff. Strengthen your core and your upper body. Stretch after.

When you're touring on a really long tour like that, you'll enjoy putting in about 60 miles/day. That's 420 miles/week, loaded. So you'd like to be able to do close to that, unloaded, first. Now, I don't do that. I have many 5000 mile years in my legs and have ridden 400k in the mountains in 18 hours, so I don't get particularly tired at a touring pace. I know I'm good to go. But you should know that about yourselves, too. I find it interesting to see what I'm capable of. I'm 65.