I've done STP the last two years, both times as a 2 day ride. Two years ago, I followed the chart provided from Cascade. I got pretty close to their suggested miles before the ride. Physically I was fine. My bike however broke with 20 miles to go...
Last year, I was only able to ride about 1/2 the suggested miles prior the ride. I didn't have any trouble on the ride.
I think using the Cascade suggested mileage helps a lot. Also if you could participate in any of the Cascade Training series rides, that would probably help. I was hoping to do that last year, but it didn't work out.
This year I'm planning for the 1 day STP. I'm pretty much in the same situation as you, in terms of getting my speed up, so I can finish before dark.
As far as my bike. I ride a steel road bike, and I do sit a bit more upright. My bars are level with my seat. For a longer ride like this, fit is crucial. My LBS did a great job getting me fitted.
I think if you put in plenty of miles before hand, have a good fit on your bike, you'll be fine.
Also, I didn't find the hill in Puyallup to that bad at all.
Finally if you can make it to one of the Pre-ride seminars, you could get some good information there. I think there is one this Saturday at the Cascade bike expo.