There's a thread going in the LD forum regarding preparedness for 300k and longer brevets, and general concensus is:
You know you're ready for one when you finish one.
You don't need to haul all your touring gear for a double metric, so you'll be rolling lighter. Plus, try using my method of getting through the ride: Don't use your computer to track total miles. If you do, the temptation is there to look at 5 or 10 miles and go "Geez, still 115 miles to go!"
For the 400k I just finished, I was resetting my computer at every turn marker and only tracking the mileage per each leg of the route. That way it's only 1.7 miles to the next turn, or for the long highway stretches, 17 - 30 miles per leg. You're never seeing that you've only gone 20 of 125 miles, or that there's still 80 miles left.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.