I've ridden STP 10 times - 9 as 2-day and 1 as 1-day. A good training "guide" is found on the Cascade Bicycle Clubs STP website on this page:
http://cascade.org/EandR/stp/stp_mileage.cfm
Scroll near the bottom of the page - the charts give you suggested mileage, increasing weekly. I haven't stuck strictly to the mileage, but used it as a guide, some weeks my miles were lower, some were higher. I found the most comfortable STP years were when my training miles were at the high ends of the suggested mileages, so these charts do help.
Some of the most important words of advice I can give is to have fun and to stay aware of your surroundings. Don't go out too fast - you'll feel burnt out later in the day. Keep smiling and have fun. There are LOTS of riders out there, can be crowded sometimes - stay aware of those around you to avoid accidents. There are probably more crashes and flats in the first 20 miles than in the rest of the ride. STP feels like a rolling party - so have fun :-)
Edna