It's called "base miles". You build your base gradually. Once you are in the habit of riding ten miles, it's not a big deal to add a couple of miles. The more "base miles" you get under your belt, the easier riding will be and then you can work on speed, if you want to go faster.

A typical training week for people who already have a bit of a base, but want to train for say a century, would be to do one long ride a week (as was already suggested). You add a little to whatever your longest ride is and keep adding a little each week until you've reached around 70% of the total number you want to do at the event. (However, the more you do before the event, the more comfortable the ride will be.) And let me just say that while I've done a lot of centuries, there has always been some amount of discomfort along the way. It's not all cozy and comfy no matter how many base miles you have (at least in my experience). Especially the more hills there are and the more you choose to push your pace. Yet, not pushing your pace means sitting in that saddle for that much longer and that's another kind of discomfort!

Yet, the more base you get in, DEFINITELY the less discomfort.

To build speed, you do at least one "speed" workout during the week. This could be intervals or tempo (research those terms if you want more info) or just doing a shorter ride at the fastest speed you feel you can reasonably maintain for that distance. If you find you can easily do a 20 mile long ride, try doing a 10 mile ride mid-week and push your pace a bit. (Get a bike computer if you don't have one.)

You don't want every ride to be "long" or every ride to be "speed", though. You need moderate paced rides in between. And you want to do a "recovery" ride after any particularly hard ride. Just get out and spin your legs super easy for a while and it will help your muscles recover faster and your next ride will be easier. You can also just go for a walk or anything that gently works those muscles.

For now you should probably just focus on distance and add a little each week, whatever you feel comfortable adding.

Having a goal might be a good way to motivate yourself. Find an event that is a 25 or 30 mile distance. Maybe something that is two or three months from now. Write out a tentative training schedule adding maybe 2 miles each week. You might be surprised at how many miles you can comfortably add as you go, though. You might go from adding 2 miles to adding 5 or more.

Also, bike fit is HUGE. If your bike isn't adjusted for your body, you are likely going to have aches and pains that you don't have to have.

Proper pedaling technique helps you get the most out of your pedal stroke, which makes your overall ride easier. And that's assuming you are clipped in, which enables you to get power from the full circle of the stroke, not just pushing down. If all you are doing is pushing down, you are going to wear out those muscles used for that motion. But if you are also pulling up, pushing forward and pulling back, those are all different muscles being used a little at a time. It's hard to get the perfect pedal stroke. I still have to consciously think about it and I've been working on it for several years.

The lighter your bike is, the easier it is, particularly on hills. You might finish a 25 mile ride well ahead of someone else of the same fitness, but you didn't have to work as hard because your bike is five pounds lighter.

There are a lot of things that play into being able to ride long distances with "relative" ease. First is just getting in those base miles.