This will be my 12th STP. I did my first in '84 and missed 2 because I went to the TdFrance and one due to injury. I started just as others said....I rode one long ride each weekend. I increased by 10% each week. 50 miles seemed like a million miles but ultimately it wasn't a big deal once i had done it several times.
I refined, (I still refine) my equipment. if it takes expensive shorts and saddles for comfort, it is worth it. A cheap short left me raw on long rides. Better shorts seemed to work better. Money doesn't eliminate rearend pain but, in general, I would pay any price to remove that kind of discomfort on a long ride. I experimented with many brands. I have, finally, the saddle I can endure for hundreds of miles. I ride a selle anatomica. www.mcwin.com I think that is it. Spendy though. Going cheap is great for short rides but 200 miles can take the joy away. Than any price is right.
I have learned that hills and short fast painful rides have huge impact on your long ride strength. Don't avoid "hard" "grinding" slow climbs. A ride from Longmire to Paradise is a good training ride. Mind you, i am not speaking as a killer powerful speed racer. I am a very slow rider, still suffer on hills, am passed by 70 year olds and could lose 20 pounds.
Another key to success, from my view, is to plan on riding past 100 miles (go to castle rock) on STP's first day, because on the second day you have only 85 miles. That is a huge mental deal.
Food, as mentioned is huge. I try to spend my time on a bike eating calories rather than laying at a rest stop eating, but that takes time to learn. One thing that used to help was to get off my bike and stretch every 20 miles. Huge difference. Last year I rode STP on Saturday and rode back to Oly on Sunday...slowly, mind you but it is more of a challenge. I stopped every 50 miles to stretch.
As mentioned previously, Merckx said "ride the bike" ....Miles of any sort have a payday. happy spinning
John