You pretty well sum up where I was a year or two ago. I started throwing in some hills on my various loops, and saw a big jump over the first few months. Since then, been looking into more alternatives to keep things moving along.

For me, i've found the most returns in longer intervals, 20 minutes at or under 1 hour TT pace. Going down from that, 4-5 minute intervals, hitting them pretty hard, getting to/over 90% of HR Max by the last one.

Some ideas and explanation here: http://www.cyclecambridge.com/top4workouts.html

Be aware, that different lengths of intervals will stress different systems. 10 x 1 minute intervals don't do the same thing as 1 x 10.

As well as the names mentioned in this thread, I'd also look into Andy Coggan's work.

Enjoy. Or, at least, enjoy the benefits, I wouldn't say that intervals are always enjoyable!

B