I use a B17S. My longest rides have been about 40 miles. After 40 miles, I feel a bit sore on my sitbones, but not too much. -About as sore as the rest of my body after 40 miles!
I have experimented a lot with saddle position. Raised my saddle several times. Started with it all the way forward, then all the way back, now it in the middle and feels just right. I also felt that with my seat higher, I was able to distribute my weight better between my feet and seat and arms, instead of most of my weight on just my seat. It's easier to put more weight on your feet while you are peddling if your leg is "almost" straight on the downstroke. Check to make sure your seat is not a bit too low.
Like you, I feel like my sit bones are right on the saddle, but it does not bother me like it seems to bother you. I don't really understand that.

How is your chamois pad? They say this pad is really good for long distances:
http://www.teamestrogen.com/products.asp?pID=12919
Perhaps you should get a pair of those for your upcoming long ride and wear them under unpadded tights or knickers. It might make all the difference you need. Might work for you. I rode a month on my new Brooks with NO chamois, and found that 15 mile rides were ok, but above that I really needed some good chamois padding. It's made me way more comfortable now on my usual 15-25 mile rides, and the 40 mile rides are pretty pain free too.

Have you Pruf-Hide'ed your saddle several times? I rub some into the under side as well around the sitbone dents areas, whereever my fingers can reach under the saddle around the saddle frame. (I only polish the saddle top when the PH is dry). I've thorooughly applied Pruf-Hide about 4 times to my Brooks. It's taken about 3 months to shape itself to me fairly well now, and I expect it to keep shaping for a while yet.