Answer: it depends.
Mostly it depends on how good your residual conditioning, or conditioning from other activities (shoveling snow, perhaps?) is.
Say you'll try the brevet if you've ridden 80 miles the weekend before (a bit of a stretch, but still reasonable). If you use the 10% per week rule of thumb for extending your long ride for each week, you need to be able to ride 50 miles now. How does that compare to your fitness?