Many people come to touring by bicycle from backpacking and it is perhaps the easy path, with lots of knowledge overlap. Keen cyclists arriving from a pure love of the bike have a whole new discipline to learn. Either way, you are all most likely to be looking for a way of extending your range using self powered travel that still allows you to experience the landscape and meet the locals without the barrier of a wheeled metal box getting in the way.
Within reason any bike will do. Add the backpack and you may find that you already own everything you need for that first tour. Take a train to get you out into the countryside and away you go. If you can, start out on that first trip from your door and camp for the night. Day one and already you can call yourself a cycle tourist or perhaps you like the new term - bike packing, and already you are part of a new outdoor trend thanks to the lightweight kit you already own. To be honest, if you want to get out onto rough tracks that only a mountain bike will manage, you may never want to look any further than strapping a bag to your back or part of your frame and embracing bike-packing.
If you are looking to ride on surfaces that are not going to shake you and the bike too much, then you have the possibly more comfortable route of bolting pannier racks to your bike and hooking panniers to them to hold your stuff. Chances are, if you're eyeing up the mountain bike you already own, it will have front suspension and the bike may weigh rather more than you would like, but lets just see if you like cycle touring first. For the first few years we did everything by bolting a rack to the back of a mountain bike and getting everything into just two panniers. Looking back I have no idea how I managed, but once again the lightweight kit you already own or are thinking of getting makes this an option and not feeling too self concise sitting down to a meal in a restaurant in the stuff you have been biking in all day helps.
If you have the confidence to set off into the hills with a base weight of 9Kg, there is every chance that you may never need any more than two panniers slung off a rack on the back of your bike. As a happy result of this low weight a few other good things will come your way. The bike can be lighter and more race specked, the rack could be thinner and lighter and the tyres thinner. You have stumbled into the world of Ultra light cycle touring and 100 mile days with camping are a possibility. Throw in a credit card, pop on a handlebar bag and Lands End to John O'Groats or a European tour taking in the Cols of Le Tour is possible. You really do not need the same bike as Mark Beaumont used to go around the world to go most of the places in Northern Europe. It may be rough, but it is going to be tarmac almost all of the time.
So, now to get you out of the door and down the road. You can do ' Credit Card Touring '. A bag, possibly slung under the saddle with your very basic essentials of clothing and tool kit. One of my big heroes of distance cycling is Nick Sanders. He has now turned his attention to motor cycle records but back in the day he held the round the world record. 170 miles per day for 80 days. He put the record out of the reach of mortals to the point where they had to change the rules. If you take a look at some images - or pick up a second hand copy of the book, you will see he only has 2 small front panniers. I have tried this and it works very well indeed. What really does not look so great are the shorts he uses. For ultra light or credit card touring 2 bags at the front or back ( the small panniers - if you have bigger ones the temptation to fill them with stuff is overwhelming - so don't ).