I managed to snag a signup for a 400 km in mid-May, which will be my first regular brevet this year, although I also participated in a Fleche in early April (DNF). I still have three weekends to prepare for the 400, not too different really from the Fleche in terms of time and distance.
Since I ride centuries year round, you can pretty much put me on a bike any day of the year and say: "Ride 100 miles or 24 hour" (or whatever distance mere mortals would call insane) and I'll do it. I may not do it particularly fast, but I'll finish it somehow.
I love the local club's 400 km course. Most of the climbing is in the first quarter. If you manage to do OK in that part, you can finish the event unless you get terrible sleepy later on. I can handle 24h without sleep as long as I've had a good night's sleep the night before and don't start out with a major sleep deficit from the previous week. I personally can protect myself against extreme sleepiness on 400 k and Fleche rides pretty well by booking a hotel near the start and having an early night.