you will be setting off soon, so unlikely to change things, so given the weight of the bike and your age, (just a bit more than me) I would suggest if possible to plan for a few shorter days at first, maybe 50, 60k, but if you can't, just make sure that you sleep well in the days prior to leaving and as you say, slow and steady.
Thanks djb for the suggestions and thoughts. I've been riding 70km every 2-3 days for a week fully loaded and will be going out again today. It's getting easier. The only thing that hurts is my rear end at the end of the ride, but I can feel that also getting better. I seem to be slowly conforming to my saddle, which is a bit weird. I tend to move around a bit always searching for a good posture and there is one position where everything is just right, little weight on the hands, back ok, smooth pedaling and the saddle just feels like a perfectly fitted hammock. This only happened in a noticeable way last time I was out.
I'm going to push the bike up an overpass today because I have as yet no idea of what it'll be like pushing for when I have to.