I second the idea of back-to-back days. The second day is always worse than the third.
In 2015 I went 3,000 miles in 23 days. I did a ton of training before that. I tried to do it all- I did a double, an Everest, and a 10-day block of centuries. That was good for my confidence, but I think I overtrained.
This June I'm doing 1500 miles in 10 days. I'm taking it easier this year. I'm just doing back-to-backs 1-2 times a month and one non-stop 400 miler. That ride is more for fun than training, really.
As long as you're in decent shape it's more about determination than anything.