Rod,
I had the same problem, on my Silvio,: immediate pain / pressure when just sitting on the bike.
I tried what Doug said (reposition the cushion) and it helped but just delayed (and dulled) the pain for maybe 10 - 15 minutes.
Interesting that you noticed it after so much riding though (and only one day off the bike).
I now have the reverse to report, this is more expected result I think.
Chronology of events:
I inserted 1/4 inch of extra cushion below the top of the seat pan (inside the seat cover) that sat me up a little higher and worked for the first 30 miles of a 65 mile ride before it also got kinda too much pressure, not bad but I was experimenting in the first 100 miles on my bike.
So I moved it to my lumbar region and that worked pretty well too, perhaps better.
That stayed that way for about 400 miles of daily commutes (between 12 & 34 miles each day).
Then one day I took it out, I don't remember why, and between 400-550 miles now I have been pain free in that spot with the stock cushioning. I don't even notice it anymore...BUT...the seat cushion top is still pulled up over that topmost seat-pressure-point.
You might try just a little extra cushion here and there?