My advice woud be to ask the tour operator. They determine the pace, or the attitude to the pace. Dreamweaver2 +1 on normal touring. I have often said, that, and the 3 day thing is part of my practice. I have a friend though who is a racer, and they did a tour across canada where they did old guy RAAM pace the whole way. I would drop out on the first day. The critical thing is the pace (and your current level of fitness). If the tour operator assumes that at least some of the folks will show up unfit, every time, then the pace may be reasonable to start. It all comes down to pace. If you live anywhere near the starting point you could just try riding the first day, over and over. If you don't maybe you can get a blow by blow of the first day, and try to relate it to a local route and try that over and over.

In general, people today are far too obssessed with training. We have people doing office work with more rigourous training schedules than people training for the olympics in the 1930s. When i was on the high school rowing team in the 1970s they did oxygen uptake test on us, and the fitness level of 17 year olds was higher than the numbers for the pros on the Toronto Maple leafs (think smoking). Training is fine if you like it, but unless you start unhealthy, you shouldn't require any special training.

The other problem people have is they think these routine activities are super achievements. There is nothing all that tough about cycling all day on a bike that allows you to maintain a constant cadence, and power level. Many people in the world would find it easier and certainly more pleasant, than their day jobs. When I did my half a continent crossing in 05, I weighed 270, and had incipient heart problems and high blood pressure, and was in my late 40s. I took normal touring load, like about 45 pounds plus food water, etc... I invariably passed folks on guided outings the few times I came upon them. Operators are in a business that involves hearding sheep at the pace of the slowest lamb.