A word of warning - short (~ 2 mile) and steep climbs are not the same as a long climb in that 8% region. I also find for myself I am not prepared for a century until I can casually do a metric. So seeing as the ride is this Sat and you have only been doing 40 milers I would seriously get my head around not feeling like I had to do all three loops.
Maybe I am reading the maps wrong but it appears loop 2 (Mcneil canyon loop) is the big challenge, I think I would tackle that first, I would also not be afraid to stop 1/2 way up the hill and take a break. If I can get through that one - and it appears there are ample flats to recover, then I would do the others. That way I know I have the worst behind me and can enjoy the rest of the ride - besides after the ride is over you can talk about your experience on that part of the ride without having to say you skipped it.

It's a little late for this but I would consider a 12-27 or 11-28 cassette for that climb, but I am old, slow and fat.