In bicycling, a century is the term for bicycling 100 miles in a day. Although it is not a direct comparison, bicycling a century is considered to be the equivalent of a runner running a marathon: am ambitious goal, but a realistic one that is in the reach of most healthy people. It is not a perfect comparison: to run a marathon takes better physical fitness, and higher cardiovascular output, while a century takes more time and therefore a different type of endurance.
Many people who ride a century go on a supported ride. Bicycling for 100 miles takes a lot of food and water, and mechanical support in case of failure, so many people find it safer to go on a ride with established rest stops and people and equipment who can make repairs. These rides are also planned along routes that minimize traffic. The downside to this is that someone wishing to ride like this has to pay, and also has to wait for such a ride to be scheduled.
I have ridden one century so far, in September of this year. I rode on my own, picking out a spoke and hub route that let me ride 100 miles without ever leaving walking distance of my house in case of mechanical failure. I actually did have a flat tire and had to coast 10 miles downhill to a place where I could get my tire fixed. Every century ride is different, and mine was done on a touring bicycle on mountain roads with a bit elevation gain. One thing I discovered is that riding uphill becomes exponentially difficult when tired: about sixty miles into my ride, I had the last major uphill portion. Those four miles with a relatively small amount of elevation gain (around 300 feet) were the hardest physically, and I was worried about my ability to do another forty miles. However, on flat terrain, while fatigued, I was still able to continue.
Many of these instances of pacing and training can be found in other places, and I am not an expert on that, and as far as cyclists go, I am not particularly athletic. But I can mention one important factor that many of the training guides about endurance forget to mention: someone who is riding a bicycling is piloting a vehicle. The basic fact of balancing on a bicycle and steering it is something that most bicyclists take for granted, especially those who have gotten to the point when they can ride 100 miles. But when riding for 100 miles, it takes a lot of mental and physical endurance to keep riding with a normal amount of control. Moving your legs for 6 to 8 hours isn't as much of a challenge as it is to keep staring at a roadway, looking out for any random rocks or potholes that might send you sprawling. Along with that, the slightest misalignment or mistuning in the bicycle, where it is in the chain, brakes, seat or wheels becomes very noticeable. And of course, getting an out and out flat tire is hard to recover from. So if there is one piece of advice I would give to people thinking of riding a century, it is to focus on the vehicle aspect of the trip as much as the exercise aspect.
The biggest reward for me is that after riding a century, my ideas of what is a feasible and pleasant bicycle trip have all gone upward. Knowing I can ride one hundred miles, 50 or 60 miles seems less daunting, and 30 or 40 miles seems like hardly an exertion at all.