Benefits of interval training?

I've just started a program that includes interval training to get ready for El Tour de Tucson in November. The program seems like a great opportunity- it's a 9 week series of presentations, training drills, and rides put on by the University of Arizona medical center and coaches from Carmichael Training Systems. Not to mention that there are some freebies, including a complete kit and free samples of gels handed out each week.

I am over 50, have been cycling regularly since June 2009, and ride about 100 miles a week or a little more. My longest rides to date have been about 65 miles, but my plans include longer rides to get ready for the 109 mile distance of El Tour My average speed is currently 14.5-15.5, but occasionally faster with more favorable weather or on flatter routes.

I was just wondering what kinds of improvements I might notice by the end of the training.

A lot! :D

My biggest improvements happened when I started following a lot of the training rides my husband was doing, given to him by a professional coach. I was doing a lot of the same events, or similar events during that time, and modified the schedule a little for my level.

I believe the number one thing that helped me develop speed was intervals. I learned, however, that I don't like it when it gets all complicated. Simple is better for me. I originally worked with this same coach a year ago and he gave me all these specifics on what exact gear to be in for like 30 seconds, moving from the hardest gear during an interval to the easiest and back down. At the moment, it actually sounds like something I might want to work on, but at the time it was too much detail to think about and simply not fun! I learned to adapt intervals on common routes I do, for certain distances, and sometimes length of time, depending. Like if there is a stop sign half a mile up from a signal, I won't be getting in a full 2 minute interval, but I can do that half mile as fast as I possibly can. Recover for a while to the next signal, then do another interval to the next signal. I keep track of how fast I do these intervals on my Garmin. I hit the lap button at the beginning and end of each interval and I can tell for sure if I'm getting faster. It's fun to see improvement, even little one's, because all the little one's create BIG ones. ;)