...My question is about targetted training, not about the expected speed of unknown companions. Most of my training will be indoors, without the variables folks with more frames of reference use (average speeds, distances covered...). In effect, I want to equip myself to generate the approximate watts required to propel a “typical” loaded bike at a sustainable (and “typical”) pace, assuming my companions are bike fit. 150 watts? 175 watts? Other?
Sorry, your initial question was quite clear, but I read too much into the question when I discussed solo riding versus riding in organized groups.
I really can't make any suggestions for indoor training. After an hour on an exercise bike indoors, my butt is sore and mentally I want to do something else. But outdoors, when the weather is conducive I often do rides of about 100 km. And a year and a half ago did two 200k brevets. Right now there is 30 cm of snow on the ground, I am not doing any long distance rides.
I am not suggesting you skip the indoor training, that is needed for muscle mass retention and a good cardio system, but I do not think there is a good way to train for day after day bike touring which is as much mental and emotional as physical ability. Long days on a bike, day after day, does not correlate to short intervals of indoor training.
I fully agree with everything Happy Feet said. I think the best thing you can do is make sure you are reasonably fit when the time comes for your trip.