Andrew R Stewart 

Good advice here all around. First thing is to start the tour in very good mechanical shape. Second is to plan on a day to service your bike every few weeks/once a month. (Cleaning including chain, lubing, checking and potentially correcting basic stuff). By staying ahead of the wear and tear curve the chance to be stuck out on the road is vastly less.

One way I have suggested many to begin to learn this stuff is to get a bike that it doesn't mater if the efforts are not good ones. Cheap used, from co op or bike kitchen or even the curb lawns on trash day. Try doing stufv to it like removing and reinstalling the chain. Same with a spoke or two (and deal with that rear cog set for the first time). Same with a cable and so on. Sure you'll spend some small amount of $ for a few low cost parts and maybe a tool or three (and if done with a repair class/bike kitchen the tools might not even have to be bought). But the understanding about the basic stuff, the confidence gained will be large.

When I tour I tend to kitchen sink the tools and basic repair parts for my bike. But I rarely have needed more then a tube because I stay up on my bike's condition before and during the tour. When I did my sort of cross country tour in 2017 I brought a spare tire and chain too. After about 1000 miles I swapped out theses with the now worn old ones. Many will say that this is a very low mileage for needing replacement. I would say two things in reply. One is that touring miles are harder on wear prone parts then any other riding. You're powering far more weight along and often leaving the bike outside for days at a time. Rubber and metal wear away at a faster clip and UV takes it's toll with no daily pauses. Two is by replacing before a part is worn out you both extend the life of associated parts as well as avoid those worn parts from causing your break down. Andy