You'll survive, your butt won't. In that heat drink often. Don't quit, day two you will hurt like hell even if you quit at 50 miles. Do the 90 so you can say you finished.

If you can, train for the ride and do a longish ride (40-50 miles) a 5-6 days before the event, then a series of short rides leading up to the tour date. Anyway, thats what I would do, may not be the best advice but I'm 39 and it works for me.