As far as how much training you really need, part of it will be goal oriented. What are you planing? Recreation? Professional rider?
I personally am a bike commuter, plus some recreation. I find cycling is very good for my knees. I've gotten away from cycling a bit, but getting back into it now. So, it has been over a year since the last "century" ride.
One year I had a goal of averaging about 20 miles a day, or a little over 7,000 miles for the year. Whew!!! And those numbers were chosen because I was getting a lot of commuting miles (riding between half and 2/3 of the days, so each ride tended to be substantially more than 20 miles).
Somebody above suggested riding 100 miles to train to ride 100 miles. But, I find that if you ride 150+ miles in a day, then that 100 mile ride gets much easier. But, if you aren't racing, you don't have to do that every day.
Nonetheless, for those occasional longer rides, there are fitness benefits from doing those 150+ mile rides.