There are tons of them. I've read many of them. They all say the same thing.
I have only been cycling for a couple of years and have done lots of centuries, training for one is not rocket science. Especially for you, since as a runner, you are pretty in tune with what your body is telling you.
Here's what you do:
Start riding a distance that is challenging for you, but doesn't leave you totally trashed at the end. Ride that distance for a couple of weeks until it feels really easy. Then start increasing your mileage in 10% increments. Every time you increase your mileage, stick to your new mileage until it becomes easy. Then increase by 10% again, etc. Do this until you reach 75miles. This should be a couple of weeks before your event. Train on terrain that is similar to what you will be riding on for your event (ie, if there will be hills, train on hills). Taper your volume the week before the event. The week before your event, get your bike ready, eat well, stay hydrated.
As you increase you mileage, you will also be increasing your saddle time. Take this opportunity to figure out what works in terms of clothing (and position on the bike, if that may be an issue). This is also a good time to start paying attention to nutrition.
In terms of nutrition, you will be burning a lot of calories and depleting your glycogen stores. If you did much distance running, you may have dealt with this before. Basically, you will need food that you can eat while riding (easy to unwrap, and doesn't cause gastrointestinal issues). The food should have easily absorbable sugars, protein, and maybe some potassium and/or salt for electrolyte replacement. Bananas work well, and peanut butter, but these can sometimes be challenging to eat while on the bike. Alternatively, there is commercially available stuff like gu, hammer gel, shot blocks, etc. Try these out on your training rides and see what works for you.
I think that really covers what you need for training for a century. And by that I mean that you won't have a problem finishing the century. If you want to be really fast, then it gets a bit more complicated. But for your first century, I would just focus on finishing it, and having fun while doing it. I have met a lot of people of organized century rides whose longest ride before hand was in the 50mile range. They all finished, although not without some suffering. If you train up to 75miles, your century will be breeze.
there's a an entire gospel of training programs but i'd personally suggest getting on your bike and just riding a century. might be a little easier if you can find someone to ride it with you as you can switch on the pulls and you'd have some company which makes that kind of time in the saddle easier to take.
What type of running have you been doing - track events or longer, road based running like marathons and the like?
If it's the later then you should already have a nice base of aerobic fitness. You probably have about as much meat on you as a butchers pencil. Both those put you in very good stead for riding a century.
Your first priority should be getting a good position on the bike and then just going out to ride. You'll probably notice less residual soreness following a training session on the bike compared to running due to the lack of impact. This and the fact that when you have a good position on the bike, which will balance your weight nicely, means you can spend less time bearing weight and more time getting on with producing power, riding fast and having fun.
If you did nothing else but just go out and ride for an hour or so fairly hard on Saturday and Sunday, to get used to being on the bike and get some good training and a couple of sessions indoors on the trainer (if it's not that nice out at night) to get some harder sessions in then you'd be in good standing within a couple of months.
If you can run a few hours then riding for five should be a fairly easy for you.
Two items that can't be overlooked - good, well fitting shorts and a comfy saddle of the correct width. I dig specialized saddles - your local Specialized dealer should have a 'butt-o-meter' to check for 'sit bone' width and shorts... well, you'll have to find that out for yourself - but you really do get what you pay for.
That's good advice. I am trying to get some training done for my first century but as I write we have 2 inches of snow outside.
The advice above is great. One thing I do when I don't have time to ride for 3 or 4 hours is make shorter sessions intense.
This will build your strength and get your lungs used to taking in huge amounts of oxygen when you hit an hill.
Yesterday I managed a quick 10 miles with a 3 mile steep hill. I didn't think I would make it to the top but I just managed it.
If you have any good hills you can use them as a guide to your improving fitness.
It is great motivation when you fly past the point on the hill you once had to get off and walk.
I wouldn't recommend you do this every week, just once in a while.
Increase your mileage steadily and train on similar terrain as your ride like CalicoCat says and you should be fine.
By the way CalicoCat. You can come and write on my blog anytime you have a spare 10 minutes.