Catrin, do the century first, then make the decision whether you need a longer ride or two before the brevet. You have to go out and do the miles and see what happens, then reassess. One example of what I mean by what happens to the body will be the wind. You can get out and do 60 miles, start early and be back before noon. Typically there is no wind or low wind in the am hours, so a metric is easy and fun to do in the morning hours. But around noon the wind starts to pick up, and depending on the weather and the region, the wind can pick up real fast and get strong and gusty. If you do a century, you will be doing the last 40 miles in the afternoon, and potentially riding into a headwind or encountering strong side gusts, all when you are getting increasing body fatigue due to distance. You need to find out what happens to your body fighting the wind for the last 30 miles, can you maintain your speed, or do you get increasingly more fatigued until the average speed drops, the time on the bike increases, the blood sugars drop, and so on. If you get a strong tailwind for the last miles, then stop and buy a lottery ticket on the way home because it is your day.

Also, on your schedule, allow time for rest before the longest rides, at least several days off the bike.