I did a 400K (251 miles) with 15,200 feet of climbing yesterday. Before that, the longest I'd ever gone was 161 miles, and that was on the dead-flat Ride Across Indiana. But, I'd ridden about two dozen centuries in the last year, many of them hilly and many of them in really bad weather. I think one critical thing to keep in mind when doing the ultra distances is to pay attention to food and water religiously. You'll also want to have a lot of "saddle time" in. You mileage may vary, but I don't think there's much benefit to "training" rides of more than 125 miles. Doing centures on two consecutive days also gives you a good idea of what it's like to be sore and tired on the bike and to keep riding. Do the longer rides for a good aerobic base, but make sure that you get in some quality time -- shorter rides -- in the hills and doing speed work. Anything much more than 100 miles takes a toll on you and it can take a while to recover.
Another critical aspect is to pace yourself religiously. Let the big dogs go run up front; most likely you'll catch them later in the day, and some of them will end up DNF. Especially true if the route has a lot of elevation change; force yourself to use a low gear, even though your heart wants you to mash up the suckers in the big ring. The longer the ride gets, the less finishing (and even finishing first) is about physical conditioning. Make smart decisions about how to ride, make sure to eat and drink, and minimize time spent at stops and you'll find that not only can you ride the long distances, but you'll likely finish with a great time and even will feel good at the end of it!