I'm with those that find commuting to be good training. I don't have a car. Commute 40 miles round trip. I don't train to tour. I don't even do one loaded ride first. I just live my normal commuting lifestyle. I just took a three week tour. The first day out I rode 75 miles. My load is probably five times my touring load. The first day was "ok". The 2nd day was harder but not bad. The 3rd day was about the same. The 4th day I felt really good. After that it all started fading into the background. I was not spending so much time thinking about the ride and how I'm doing. That was all more and more incidental and just an unconscious side effect of exploring the world.

I also identify with a comment I saw here about day 14! After that I was really on auto-pilot. Never thinking for a minute that I might run out of energy or something. I also learned to keep moving more. Sure, I took breaks when I wanted to. But slowing down, coasting some, scooting around in the seat, doing lazy figure 8s in a parking lot for a minute are about as much rest as stopping but don't kill your daily mileage nearly like out rite stopping and doing 0 mph.