Last year we rode 200+ pretty much every weekend, as soon as the snow cleared, because we were on a mission, but we only picked rides that were well within our capabilities so that we didn't have to work very hard and recovered quickly and easily, because the whole point of the season was to keep riding.
This year the goal is different. We will probably do either a 200 on solos between them just to keep the legs moving, and take it proper easy (12 hours, pub stops etc) or travel to (say) the Peaks and do something shorter and hillier. We need the practise, there's no hills round here and we have to go looking for them.
Make sure you get plenty of sleep, for recovery and to minimise sleep debt going into the 600.