But Ron, all your double centuries were timed - you had to make it to the finish within the required time (was it 17 or 18 hours?) or you wouldn't have received credit towards the CA Triple Crown.
For brevets, you likewise have to finish within the time limit. For a 200K, the time limit is 13.5 hours; for a 300K, the time limit is 20 hours. Doesn't matter if you finish 5 hours under the limit or 5 minutes under the limit. Either way, you've successfully completed the brevet.
( I don't remember what the time limits are for the longer brevets, since I doubt I'll ever do more than a 300K. )
In case you haven't figured it out yet, you have a much longer time on a per mile basis to complete a brevet than a double century. So don't worry about that.
But do bear in mind that brevets are essentially unsupported. There is usually water and maybe some food available at the checkpoints, but brevet checkpoints are spread much farther apart than the rest stops in a typical supported double century. It's up to you to carry your own food or be prepared to purchase it along the route, and carry whatever other stuff you might need (lights, raingear, etc.)