I'd suggest you pick a day in the very near future and see how far you CAN ride.
That would actually give you a clue if it's doable.
Doing 180 miles is far tougher than doing 20 miles 9 times.
There's things like lactic acid build up and you have to know what to EAT, to replace minerals etc. that help your muscles to function.
How's the terrain? If you have to do a bunch of climbing, even if it's gentle, that can knock you way down energy wise.