General advice for tapering is to reduce the volume but keep the intensity mix about the same. I'd have thought 7-10 days taper for a bike-only event would be about right, running needs a longer taper as long runs take more of a toll on your body. Everybody is a bit different though in terms of how they respond to a taper. If it was me I'd do something like:

- 14 days out, do your last 100+ mile training ride
- 8-13 days out, still do your spin and HIIT classes pretty much as normal, but if there are any longish sessions with a lot of intensity which normally leave you feeling absolutely buried, you might want to back off a little on these towards the end of the week e.g. skip the last couple of intervals or do them at a lower effort level than normal
- 7 days out, reduce your long ride to maybe 50-70 miles, same pace/effort as normal
- 4-6 days out, do your spin and HIIT classes, but aim to reduce the amount of high intensity work in each session to about half of what you'd normally do.
- 1-3 days out, no more classes but still aim to spin the legs out every day. E.g. 20-40 minute rides done mainly at zone 1/zone 2/recovery pace, but with some short higher intensity bursts just to get the muscles moving and remind yourself what it feels like.

Obviously also need to pay lots of attention to looking after yourself outside the training. Good nutrition, sleep, hydration, maybe treat yourself to a good sports massage, though you don't want any really deep tissue massage too close to the race as it might leave you with some soreness.

Thank you so much! This is incredibly helpful! Gearing up for my first double century...will be my longest ride ever, and at altitude nonetheless. (I live at sea level, eek.)