Horrible Hundred

I was going to do the 70 but decided to focus on running instead.

It's nothing to add mileage and lots of difficulty to the 35mi ride.  For instance, to add ~7.5 miles and include the 3-Sisters (Buckhill Rd) and Sugarloaf Mnt, when on 455 instead of turning left on 561, keep going straight until you get to Buckhill Rd (1.5mi) then turn left.  Ride the sisters to Turnpike Rd then take a left and continue until you get to 561, then turn right.  Head up to Sugarloaf Mountain Rd and take a right to do the killer climb.  Once you get to Buckhill it should be marked as the Buckhill to Sugarload to 561A is all part of the 70/100 mile ride.  When you get to 561A after Sugarloaf take a right to get back to Grassy Lane Rd (1.3mi) for the return leg.

When I did the the Assualt on Sugarloaf, there were riders training on their own in all directions that weren't part of the Rec ride.  I don't think anyone would care if you added some like the above to the ride.