The reason to ride a 200k brevet is to get into shape to ride a 300k brevet. You'll need to build your endurance and strength gradually. Increasing your abilities in 100k increments is a challenge.
The Mt. Adams SxSW 200k has 7,800 feet of hill climbing and it's profile looks vaguely similar to the Oregon Barlow Trail 300k on July 25, 2009. The elevation gains for the two rides are roughly proportional. The 200k has 3,900 feet of climbing per 100k distance while the 300k has about 4,300 feet of climbing per 100k distance.
For new riders it might also be useful to know the events are spaced between 2 and 3 weeks apart to give you a chance to recover and rebuild your strength for the next event. If your lucky the next event will be challenging but you will be able to complete the ride because your increased endurance and strength.
Barlow Trail 300k Profile
Barlow Trail First 108 miles
Barlow Trail Last 80 miles