the best resource I've found is "Cycling After 50" by Joe Friel. Great introduction to training, use of heart rate monitors, etc. Specific guidance for ramping up to a century.

that being said, I don't want to be discouraging, but listen to your body. I was in your shoes 1 1/2 years ago when I started riding after a 20 year hiatus. My goals were to ride a century and lose 50 lbs in 2006. Neither happened. My body had it's own timetable. For one thing I found that it took a full year to develop cardio endurance. I did get to where I can routinely do metric centuries, and where I dropped inches off my waistline (but very few pounds).

So set goals, but be flexible about adjusting them if necessary.

Welcome back and good luck!!!