My best years of riding brevets have followed the pattern where I don't ride a whole lot during the week (i.e. 20 or so miles a day) but then I ride a longer distance on the weekend.

So my schedule looks something like this:

Weekend 1: 200K brevet on Sat; short spin on Sun
Week: 20 miles a day, with a day or two off
Weekend 2: 100K on Sat; 75K on Sun
Week: 20 miles a day, with a day or two off
Weekend 3: 160K on Sat; 75K on Sun
Week: 20 miles a day, with a day or two off
Weekend 4: 300K brevet on Sat; short spin on Sun

Or something like that.

I will be riding my 400K on May 27th, and so this weekend I'm planning to ride a metric century on Sunday and an imperial century on Monday ... and then keep it shorter during the week.

When you get up into the longer distances two things become quite important: regularity (getting out there, even for short distances, on a fairly regular basis); and rest (taking a couple days of each week, or just short spins on some days, so that your muscles and joints etc. can recover).