Today I changed the 40/20 intervals that I've mentioned before into 45/15 intervals: 45 seconds of riding hard + 15 seconds of riding easily, etc. It seemed to go pretty well, so I'll keep the 45/15's for the next few weeks. Then, after a "rest" week, I'll try going hard for a full minute, and easy for 30 seconds, or something like that.
It's also time to start pushing for a higher sustained speed during my "pace" rides. Today I touched 19+ mph while pushing hard, but I'm a long way from being able to maintain such a pace. I do get the impression that I've become a bit stronger. Maybe, some day, I'll be flying down the road!