Next time you are by a book store, check out
The Lance Armstrong Performance Program: Seven Weeks To The Perfect Ride. I used it as my beginner training guide when I was in the same place as you. It gives a lot of great information about cycling, nutrition, cycling training and specifically answers your question about how to properly target your cycling training. From there, it recommends beginner and intermediate training plans built around the seven week time frame. Most of the beginner workouts end up being right at the time and intensity that you are looking for.
The book details a 'field test' using a couple of 3 mile maximum effort (I think I'm going to puke when I finish this) rides with a recovery period in between, to identify your maximum effort average heart rate value. From there, you use this heart rate value to dial in the rest of your training intensities. If you don't have a heart rate monitor, you can kind of end up being able to target your training using perceived exersion levels, but it won't be nearly as effective as working with heart rate data each training ride. So if you don't have a heart rate monitor, you may want to think about getting one. They start at about $25.
I was really really really suprised at how much benefit I got out of following this training approach compared to what I had been getting from riding on my own. I still use this same approach in my training now and am stilling getting great results.
Hope this helps. Good luck!
