Fresh, that is very good news!!

One thing everyone should keep in mind when comparing Brooks B17 "S" and "non-S" models:
"S" models are sometimes called women's model (though some men use them as well). It means the back part where your sitbones sit is about an inch wider to accomodate wider sitbone distance apart. It ALSO means a shorter nose on the saddle. About an inch shorter than non-S B17.
So...
A non-S "regular" B17 has a longer nose (just the thin part) and is an inch narrower in the sitbone back part. Because of the longer overall length, it also has correspondingly longer rails underneath than the S, by about an inch. These longer rails give you more range in your forward/backward saddle position possibilities. Just good to know.

A Brooks B68 comes in S and non-S models as well, but there is a difference. ALL B68's are wider in the rear sitbone area, the S and the non-S as well. They are both wider (210mm) even than the B17S (170mm). The only diff in the B68 S & non-S is the nose length. The B68S has the shortened nose and it's corresponding shortened rails underneath. B68 saddles are good for people with wider pelvis/sitbones, like me! I have ordered one because after 6 months I still find my B17S a bit too narrow for my sitbones.

Suzie, I know you are going to do your long upcoming ride wearing two chamois, but I am going to suggest that you might consider removing one chamois mid ride if you start having lots of pain. I know this sounds illogical, but...I had suggested a double chamois when you were going to ride a huge ride with a brand new Brooks. Now that you've ridden many miles on that saddle, you may want to eliminate the second chamois to give your sore/sensitive area MORE SPACE so there will be less pressure on it. Give it a try- you can always put the second shorts back on if it doesn't work well. Let me know how the ride goes- and don't kill yourself just in order to finish if you are in bad pain.
I know you are a very slender tall girl, and like FreshN, you might do well with a narrower overall Brooks model, especially one with a very narrow mid section and nose area.