I'll tell you a story, I signed up for the Grand Tour. Then I realize it had 8,000 feet of climbing two week before
So it really depends. The only two I can recall training specially for were Ride around the Bear(100 miles - 10,000 feet) and the 300K I did at Cayucos(another 5 digit climbing affair)
That being said, if you do have a goal ride in mind, it doesn't help to get prepared , so you can enjoy it. I usually try to do a metric a week(weather permitting), and found that I can usually do a 200K with much complaints.
It does help tremendously to have a bike you're happy with.