I'm thinking you can do it at your current level of training. At least, if someone asked me to do an 80 mile ride tomorrow, I wouldn't hesitate, even though I haven't done that many miles in a day since... oh, I guess it was September. (I'm 47).
Your neck will probably be pretty sore by the time you finish; bring some of your preferred analgesic.
Be sure to eat well the day before, eat a big breakfast, and try to eat more or less constantly on your ride.
If you can go for a 50 mile this weekend, and the same or longer next weekend, you'll have 100% confidence. If you can't do those rides, don't sweat it, I'm sure you can do the 80 as is.
I agree. Unless the ride is very hilly or you start out with the go-fast group, I think you should be able to do it and enjoy yourself. I'm w/ RHM in that I haven't ridden 50 miles or more since August, but I'm crazy enough to tackle 80 this weekend if I had the opportunity (and I'm 49!), particularly if I lived in SoCal! It is supposed to hit 40 here in the Boston area on Sunday, but I think to do 80 miles and not freeze, I'd have to not stop. That might not work.
Neal