Anyone who is serious about an endurance sport schedules one day a week as a "long" day.
They can be runners, cyclists, swimmers, triathletes...it doesn't matter. The philosophy behind the long day is the same.
What is a long day? It is simply a day when the training is for a distance that is much longer than the usual distances that one undertakes during the rest of the week's training sessions.
There are no fixed guidelines on how much longer the distance should be compared to a normal training day. It depends on the fitness of the athlete. A long day for one athlete could be a regular workout for another. It could also be a completely insurmountable distance.
The long day should ideally be a distance that is short enough to be within the capabilities of the athlete to complete, but long enough to be a challenge. Challenge in the sense that the athlete must moderate their pace considerably throughout the distance and must hoard strength and parcel it out over the distance. In other words, it is a simulation of actual race conditions.
Training during long days, naturally, is incredibly time-consuming. For that reason, most people have theirs on the weekend. I hold my weekly long run on Sunday evenings, if I am training on shorter distances of 14 miles or less, and on Sunday mornings (6 am) for longer distances. [This mostly stems from the fact that I am sooo not a morning person and wake up early only when absolutely necessary.]
What is the philosophy behind the long run (in my case, it is a long run, rather than cycle or swim)?
Most people will tell you that it's a way to push back the limits of the mind and body. It's only during the long run that a person gets to simulate what actually happens during a race like a marathon, and the long run prepares the body physically and the mind psychologically for what will come on race day.
I like to think of the long run as a gateway to what I call the Zone. This is not the same Zone that many amateur and professional athletes refer to (which to them, is an occasion when everything seems to be in flow and performance is optimal). Instead, to me, the Zone is that place after I've run about 18 miles. The Zone lies beyond the Wall.
Hitting the wall is a phenomenon every endurance athlete has to contend with at some point, and to perform during a race means to be able to run well even in the Zone. For obvious reasons, most of us don't spend a whole lot of time in that place I call the Zone, even those of us who train for races, but it's important to be able to abide in that place. And to do that, one must train in the Zone. It's probably not possible to feel entirely comfortable in the Zone (unless you're a connoisseur of pain or you practice exotic techniques like TM to shut out pain and fatigue).Because of the routine of the long run, sometimes running a race like a marathon is merely a formality that happens to be the culmination of many months of training (unless you're a competitive athlete whose actual performance on the day itself matters a great deal).
The reason why I say this is that for many serious runners, performance on race day itself has already been "baked into the cake", so to speak, from the many previous months of long runs and other preparation. Indeed, when people say that they admire the persistence of marathoners (and other endurance athletes), I like to say that persistence is not the most important trait of a successful runner.
Persistence only gets you from the start line to the finish line on race day.
But it is discipline that gets the runner to the start line in the first place. The discipline that allows a retiree to re-focus on a new goal after retiring, an office worker to put on their running shoes after a long day of work or the stay-at-home mother to make time to train even with three kids and a household to manage.