I know that in running we usually taper for two full weeks, but this is mostly because of the intensity and impact of running-- things that are not likely to occur with this type of riding. How should I taper for this ride?
I don't think the concept of tapering applies to endurance cycling.
It is true that you should probably "rest up" and avoid hard riding for several day before the event.
If there is one area of preparation that could really help you - it would be eating and sleeping right the week of the ride.