The definition of 'long distance' for you or for me or for someone else will always be completely different.
If you talk to a non-cyclist, 20 miles sounds like a long ride, but for an enthusiastic club member 100 miles might be a long ride.There are cyclists who regularly cycle much greater distances than this (Tour de France stages are often 140 miles or more).
For the sake of this article I'm thinking that a ride of four or more hours in the saddle is long, five or six is very long. Few of us have the time to ride for more than five hours in one session very often, even if we are physically able.
The intensity at which you cycle a long way is also important. Cycling a 'century' (100 miles) with a group of enthusiasts at 20 miles per hour is not the same as covering the same distance oevr the course of a day, stopping to take photos and have lunch etc. The first is not necessarily harder, but is certainly different.
As with all cycling, the key is to aim for steady progress in the run up to your long ride. If you usually cycle 25 miles maximum, you will find it very hard (probably impossible) to maintain anything like the same speed for 75 miles.
A significant part of the challenge, apart from leg fatigue, is that the sheer act of staying in the same position for such a long time is not easy. Aches and pains in your neck and back are common. So key to the whole practice is remembering to stretch, sit upright, cycle short distances standing (off the saddle) at regular intervals - ideally before the pain sets in. Regular arm rotations and shoulder stretching work wonders for avoiding the pain. Move your head from side to side and round and round. When cycling in a group I usually wait until I'm at the back of the group before adopting these ridiculous poses and stretches to avoid looking daft!
Over and beyond the normal challenges of training - strength, cadence, hill-climbing etc. - long distance cycling has the big extra feature that the training really consists of spending a long time cycling. The only good training for cycling a long way is to cycle a long way!
This is harder to fit in to a weekly schedule than most forms of training. Most of us can find an hour for an intensive training session, or a couple of hours for a decent hilly circuit, but regularly fitting in five hour rides as well is, well, time-consuming.
In reality it is very hard to increase distances covered by more than about 5-10% a week without suffering from overtraining or from injury. Second, as a one off you can usually manage about 25-30% more than your 'normal'. So if you plan to do a long ride, say 75 miles, in August (sponsored ride, charity ride, cycling holiday...) then you are aiming to be used to cycling 60 miles when the time arrives. Counting backwards this suggests that four weeks before that you should be happy cycling 45 miles, and eight weks before you should be confident at outings of 35 miles.
So you can see it is never too early to start planning! This will be much easier if you have cycled significant distances in earlier years, because your muscles and posture will adapt more quickly. If it your first time on a bike and you've just been cajoled into a long charity ride in a few months time - get training now!
Two of the major considerations for cycling a long distance are liquid and food. Carry at least two bottles of water - and still you will need to find more water, especially in hot weather. For food, typically your body only stocks enough 'fuel' for 1.5-2 hours of effort, and you will use perhaps 750 calories per hour (all numbers extremely variable, it's the principle that counts!) so you will need to carry or have access to sufficient resources to fill the gap - gels, energy drinks, dried fruits and a whole range of products are available. You will need them. Do not attempt to cycle for hours without food, your energy resources will run out and you will 'bonk' - lose all force - and soon be unable to continue.
Cycling a long distance is challenging, but with a little bit of planning, and a lot of hours in the saddle, in no time at all you will be casually mentioning to people that you just cycled 80 miles, and enjoyed it into the bargain!