What advice do you have for training between the brevets in a series?

This is my first season of randonneuring. I have been riding 100+ miles a week (20 or so miles a day) during the summers for many years (I am 60) with my longest ride being 85 miles. This spring, I have finished a 200k and a 300k brevet and have my sights set on 400k and 600k. The brevets are 3 weeks apart. There is good advice for preparing for a first century which I - sort of - followed. After the 200k, I simply did my usual commuting and errands on my bicycle, about 10 to 15 miles a day, with some additional riding, the longest being 30 miles. I did better on the 300k than I had on the previous brevet. I took it easy this week and now have 2 weeks before the 400k.

Any suggestions for training for this next level?

Rafael