How do you train for a 300 mile ride?

How do you train for 300 miles? I received much advice raninging from "just ride as much as you can" to "train like you would for a century". Either way it means I am spending many many hours on my bicycle. Generally my training includes two "short" rides during the week (20 miles) and a long milage day on the weekends (50 to 70 miles). Today was my long milage day. Sixty-two miles completed in 3 hours 45 minutes averaging 15.9 MPH. The last half was ridden into a head wind. The excitement when I finished was wonderful! I DID IT !!!! Accomplishing a goal feels good.
I am training in northwest Ohio. Northwest Ohio includes some of the best farmland in the nation. And it is flat. I use to live in central and northern Michigan where rolling hills often made up the route. You may be thinking "the flatter the better". At first that may make sense. But a flat route requires that I supply all the power. No coasting down hills along a flat route. Of course, no struggling up hills either. Training on one type of terrain doesn't mean I'll be prepared for all types of terrain I may encounter between Cleveland and Cincinnati.
Life is similar. If life didn't contain some bumps in the road I wouldn't be prepared for all types of situations. Life is not always smooth (anagolous to flat) and if it was I would have to provide all of the motivation (power) to move through each day. The trials in life have taught me a great deal and strengthen me in a number ways making me better prepared for life. Just like hills are necessary for training so are trials necessary in life.