I went into Vancouver Island's Hell Week like I was about to do a somewhat relaxed 1200K. It was in April, which was a bit of a challenge, but I attended a tough spinning class twice a week and followed it up with running on the treadmill and weightlifting on those days. I rode my trainer fairly lengthy times several times a week, and I tried to get out for fairly lengthy rides, including 2 centuries in March.
And I had a plan of rest and recovery on the days in between the rides, involving lots of eating and sleeping.
Unfortunately, my best laid plans fell apart when I crashed on the 200K. I finished that ride, and attempted the 400K a couple days later, but I just couldn't do it with all the bruising and road rash I had everywhere, and a separated shoulder. I pulled out of the 400K at about 260 km.
If I were to do that sort of thing again ... and it does appeal to me ... I think I'd like to have ridden at least a 300 km ride or brevet two or three weeks before the event, and preferably a 400K.