A long trip is wonderful for a couple, and it will also find every small friction you have learned to ignore at home. Months in each other's company, in unfamiliar places, with decisions to make every day, is a lot. A little planning keeps it joyful.
The single best thing we do is take time apart. Nothing grand, just a morning each, a few times a week, to do our own thing. One walks, one reads in a café. You come back with something to talk about, and the day feels longer for it. Togetherness all day every day is not the goal. Choosing each other is.
Divide the jobs and stop renegotiating them. One of us handles the trains and the bookings, the other the cooking and the markets. Knowing whose call it is removes a surprising amount of low-level friction. Swap the roles on the next trip if you like, but settle them for this one.
And lower the stakes on the small decisions. Where to eat, which way to walk, whether to bother with the church. None of it matters enough to spoil an afternoon. The trips we remember most fondly are not the ones where everything went to plan. They are the ones where we were kind to each other when it did not.