from my experience, doing supported trips are a wonderful introduction to touring, simply for all the reasons that are apparent--they take care of the route, lodgings, food, carrying your gear, tech support if you have a problem, pickup support if you cant make the assigned route--its a win win for everyone.
it gets folks out who would be hesitant or intimidated on doing it on their own, you get real world experience riding day after day which can help with making sure that your bike fits you well and your body works well on it day after day (hands, bum, feet, you name it) and you will gain confidence from the whole experience.
Ive done personally planned trips going back 30 years, but my wife was never keen on it, so for years we did week long supported trips and for all the reasons mentioned, it was perfect for her. She has improved as a rider immensely over the years, and finally last year we did a solo trip, and it went well, partly because of the years of doing those supported trips.
An important factor with signing up on a supported trip is that its a really good kick in the keester for you to get out there and prepare yourself, ride more and gradually do longer rides--and this is key, gradually increase, and more importantly, regularly ride, and this is great for getter fitter and more comfortable on your bike for longer periods.
A shout out to the Montreal based "Velo Quebec" supported trips called "Le Grand Tour", week long, very well organized, diff distances planned out per day--a really great organization. This is the one we have done 7 or 8 times, and it seems that more andmore Americans come and do it each year.
easy to look up and find the english version of their web pages.
while yes there are lots of fit roadies (fast) folks who do it, there has always been a fair number of regular folks, of all ages, on regular hybrids or whatever, and there are always folks in their 60, 70s, heck even one or two in their 80s, who do it every year.
a nice atmosphere, not competitive, and a great support system.
so, plug for that over.
the main thing of wanting to do touring on your own is to be comfortable with riding, comfortable with route following, etc etc, so if you arent there yet on your own, do more supported trips, they are a really fun way of getting out there, but nto having to worry about the logistics.
but yes, you still have to get your keester out on your bike and ride, fitness and endurance doesnt come with any magic button.