You'll be surprised to find how spending time in the saddle can increase your endurance. I know that sounds like stating the obvious but generally, you can do on a weekend the number of miles you do across the week. So if you ride 15 miles a day Monday through Friday, you should be able to ride 50 to 75 miles on Saturday or Sunday. Once you do that, increase your weekday mileage a bit, and you can increase your weekend mileage.

There is a section of bike forums for long distance riding. Some people ride 200 to 600 km in a single weekend, some even more. Those courses are not flat.