12% average grade? Whoa! Kancamungus pass gets steeper every year
The climbing itself, going up on the west side is about 5 miles at about 6% grade most of the way with maybe 8-9% near the top, From the east side it is about 3 or 4 miles of climbing at probaly 7-8% grade with maybe a 10% grade just before the top. on the inside of a uphill turn you might find 12% pitch but certainly not on average.
In 1998 I rode it from the Conway side with 70 lbs of loaded touring bike and got to Lincoln inside of 4 hours.
I rode it with a group last fall on my 25# MTB from Lincoln to Conway and back the same day.
the effort in one direction(32 miles) is about equal to the effort of a 65 mile flat to rolling ride.
Bonking on a 32 mile ride because you didn't bring enough water would rate as pretty dumb. so bring at least 2 20z water bottles