+1 on doing back-to-back days with shorter rides.

+1 on doing shorter rides with roughly equal climbing as the century that is as steep or steeper.

The couple main things I'll add, if you already ride regularly on hops over 15 miles or more (3X a week), are:
- Bike Fit
- Personal Overall Fitness to stay on the bike

At least for me, the struggle I had with distance riding for a century or beyond, were due to being unaware about my own optimal cycling geometry and my overall body fitness, including upper body and not just my legs.

Of course, I initially went through issues of poor diet and hydration in my early club rides over 50 miles. As a novice, I had to give up on a few rides because they just burned up my reserves, I hit the wall, and didn't feel like eating/drinking anymore. So I've learned the hard way that I need to eat constantly and drink electrolytes and know that the slug of energy isn't going to be available that fast. Sometimes 40 minutes - 2hrs after I eat, I get that energy. So if you didn't take that break earlier and pace myself and consume the necessary calories and intake that salt and potassium - well, I'll pay the price down the road.

But once I got diet/hydration down, going beyond 70 miles was completely unpleasant over two main things. The first for me was bike fit. I noticed that I could ride my heavy modified mtb with drops and fat slicks and go forever and still feel fresh but given a road bike, I might be able to power-climb for 50 miles, but push past 60 and my joints hurt. And that's when I found that small shifts in geometry made the mtb with drops a better geometry for me. After tweaking my road bikes for a similar fit, I found them even more comfortable and effortless to ride.

And I also noticed that I lifted weights and worked out on upper body and core, I was much more refreshed after a long ride and not beat up anymore. So I think working out the upper body and core abs is critical to ride well on a century.

Otherwise, once I had those down, I could simply ride 20 milers for a few times a week, and wait for the weekend century and it was no sweat.