Having recently skim read 3 books on cycle training and being just over a month into my training program I now consider myself Britain's foremost expert on this
I've gone from feeling like death after 15 miles to doing 40 miles relatively comfortably by doing an hour two week nights just below anabolic threshold ('fairly' hard just before your legs start to hurt / 155bpm). And increasing Sunday rides by 5 miles each week. This is all road though.
What I've picked up from the books is that there's slow twitch (weaker endurance) muscle used for long riding which uses mainly an unlimited supply of oxygen and barely fatigues, and the fast twitch (stronger shorter) that uses limited food / fat and depletes you of of strength and energy pretty quick.
The key point is to go for longer you need to keep the 'free' energy from slow twitch in a bigger proportion as possible. That means higher cadence and increasing aerobic efficiency and threshold - so you can go faster before going anaerobic and using non oxygen energy.
Riding harder is just strengthening your fast twitch and sprint speed, you should train aerobically which is about 80% max for best results.
Although I agree with some of the interval comments, there seems to be evidence that improves anaerobic and speed/power. Did a session on the turbo last night and it was much easier than riding at a constant speed for an hour too.