See a professional about bike fit. Riding shouldn't cause pain except when doing hard intervals, and in that case your legs recover a few minutes after slowing down.

Saddle and shorts which don't match your anatomy can cause problems with sore spots and chafing. Saddles with a kicked up tail stopped working for me so I switched to a flatter model. I wasn't comfortable after a few hours riding new fangled shorts with squishy padding although flat synthetic leather was fine and EIT chamois with 120kg/m^3 foam beneath the sit bones are awesome.

Lumpy bar tape or gloves can cause numbness and pain in your hands. A few fingers go numb if I wrap cork tape in a figure 8 to completely hide the bar with my Campagnolo Ultrashift levers. One wrap with a short piece of tape that doesn't look as nice works great.

All these things are solvable problems.

Note that fitness and endurance are about average daily riding effort, not your longest ride. You'll be more than twice as fit on 20 miles every week day with a 40 mile long ride on Saturday than a 70 mile once a week. Your daily average is twice as far, and you'll be able to ride with more intensity for a greater total.

I wouldn't try to ride a century on less than 100 miles a week, and haven't ridden one on less than 125.

Ride frequently, with one day a longer ride twice as far as the other days. Increase your total time riding 10% 3 out of 4 weeks if you're young, 2 out of 3 weeks when you're over 40.

You need to pace yourself to maximize distance. Provided running out of muscle glycogen doesn't stop you sooner (at best, I have 3 10 minute intervals totaling 30 minutes 15% harder than I can manage for 1:15), fatigue is proportional to the square of intensity. Ride with 1/2 the intensity and you can go 4X the distance.

On rides over about five hours, maximum average effort is limited to your aerobic threshold. You're past that if conversation doesn't flow or your legs are burning at all. Probably not coincidentally that heart rate matches the Maffetone formula for me.

Stay below that level. If you're heavier and/or riding in steep hills that will take lower gears.

You'll need to eat carbs to avoid running out of steam. Since you can only digest 200-300 Calories per hour and even that much may give you digestive troubles, you'll want to eat a little constantly on long rides.

I eat 10-13 Calories/mile on rides over five hours which is 30-40% of the total and matches the carb requirements.

I like Clif (peanut butter, pomegranate chia, blueberry crisp) and Clif Mojo (peanut butter). Anything easily digested that's mostly carbs will work. Some people like dates.