Incorporate one long true-endurance ride a week. That's 3-4 hours at a time. Here's what to concentrate on for endurance:
1. nutrition. Right from the start, eat 200-300calories/hr of mainly carb drink/gel/bars and 500-750ml/hr of water. You will ALWAYS burn off calories faster than you can digest and this is the major limitation to endurance. Once you start getting close of the edge of depleting all your muscle-glycogen, you're close to bonking and calling the broom-wagon and there's absolutely no way to recovery from a bonk without quitting.
2. pace yourself. It's hard to do when you've got 4-6 hours riding ahead of you. But each and every time you push hard above your average speed for that ride, you're causing unnecessary strain on your muscles leading them close to fatigue and cramping. It's especially hard with hilly terrain to restrain yourself and take it easy. But you MUST take it easy. If you're aiming for a century @ 15mph and 60% of max-exertion, you must let a 25mph group go as they fly by. And going up the hills, do not make an interval out of it, stay at 60% effort the whole way.
3. use easy gears. Again, you need to practice pacing yourself for hours at an easy pace using easy gears. The higher the peak muscle-forces exerted, the faster and easier you get to fatigue and cramping. Muscles applying higher forces also uses more glycogen than fats and you burn up your glycogen stores much faster.
The key to endurance and long-distance riding is more mental discipline than actual pure fitness. It's about practicing nutrition, selecting easy gears and pacing yourself.