Taper?
I am doing 400+ miles in 6 days in April. I have done this one for years now. I have done tours with more miles in less time.
I never bother to taper. I suppose you can but this is not a one day event. So I think the benefits of tapering are pretty limited.
Quite a bit of the trick on these rides is pacing. Anyone in good shape can hammer on a long ride pretty well and often ride hard on a long ride on the next day, but what about the next one and the one after that?
I have seen people come into camp on the 3rd day of rides and collapse. They miss dinner and the next day they are done. The trick is to not get into the situation where you get beaten up more and more each day to the point where your riding goes into a downward spiral.
Of course, you have to train for these things. Try to get in some longer rides before hand like 70+ miles and better yet put in a couple of those back to back.
Interval training is decent for increasing your power. If you do not push yourself in your training, you will not get faster. But do not limit yourself to interval training. Short fast bursts really do not cut it on a 90 + mile ride. Being able to maintain a pace over that distance is much more important. So train at some longer rides.