In theory, someone who starts training this weekend (44 miles) could still ramp up their mileage in the next 8 weeks and get to 85 miles. Since really the average for the week is about that (one day is around 45 miles, 2 days are over 100) , getting to 85 miles would get your body knowing what long mileage days will feel like.
It would look something like this:
This weekend's Expo: 44 miles
wk2: 48 miles
wk3: 53 miles
wk4: 59 miles
wk5: 64 miles
wk6: 71 miles
wk7: 78 miles
wk8: 86 miles
wk9--tapering-- ride a very short ride, the following weekend will be ride out.
The trick is then to increase rides quickly (think end of April) and do them back to back. Maybe even take a Friday off of work and ride 3 days in a row. (shortest mileage on Friday, longest mileage on Sunday-- or even mix up Sat and Sunday).
I'm not saying that doing it this way will be easy, absolutely not at all!! But at least it gets your mileage increased gradually (about 10% each week). Starting earlier would have been the best bet, but starting any later than this week will be much much more difficult and almost impossible to do comfortably.
The time is now. Or hold off and start again next year!