I did it in one day my first time with no issues. You are already getting good base miles, you'll want to do some long rides leading up to STP. My training for the ride was my daily commute (about 36 miles RT at the time) and several weekend rides in the 50 to 80 miles range and then I did the Cascade Flying Wheels century plus I added another 30 miles on my own (basically ride to the start and home from the finish), and one other hilly ride of about 120 miles on my own.
I was tired at the STP finish but there was never a question in my mind that I would be able to finish.
The course is flat and if you are riding around Newcastle as your profile indicates, you are no stranger to hills. Some people do it every year. I'm happy I did it, but once was enough. Enjoy the ride!