Thomamueller is right - don't rule out steel, although a really good used steel frame is hard to find. My century bike is steel - my climber in CF.
Now to your question:
1) Buy Joe Freil's book - "cycling past 50" it will help. It is tedious in spots but there is some good material in there.
2) Ride at least 4 days per week - it takes that much to get better
3) Rest at least 2 days per week - you won't get better if you wear yourself out.
4) Ride distance at least once/week
5) Get comfortable with a metric - you don't need to train for a century by doing a century, just be able to easily to 65.
6) Learn what works for you as far as nutrition & hydration. These are very important on a century - learn to drink more than you think you need and eat at least 300 cal/hr.
7) For your first century don't be focused on speed - learn survival skills.
8) Learn to pace your self, if you are compulsive you may want to start out too fast.
9) Learn to draft other riders. On an organized century there are a lot of other riders, eat from their plate first and save your reserves for later.
10) Most importantly: learn to ride your own ride and not get pressured by others.
If you are riding about 20 miles at a stretch now you should be able to get prepared by September.
This is the best I can offer up. Others I am sure can give you more.