Tours are suitable for all adults with some cycling experience, and are more into a touring style not racing.
It’s not the distance of the tours - it’s more the time spent on the saddle, usually about two to three hours, then a tea break and rest. Then on again for another couple of hours and another refreshment break.
The day can be split up into manageable sections. Time to stop and take a few photos, have a drink and most of all, time to enjoy the scenery and serenity of being free. So a 90/100ks day is quite achievable, without having the daunting thought of having to cycle a long distance in one hit.
When I rode around Australia it took all day to ride 100/120ks with frequent stops just to enjoy the moment, therefore I didn’t have the feeling of exhaustion at the end of the day.