There are two things that I think have put my cycling fitness over the top, the first is consistency, just riding, period. Since I'm (and have been for 30-years) a cycling commuter, I have a lot of consistency in riding. The second thing (and I believe it's the icing on the cake) are intervals of all types, including all-out sprints from a dead stop. My cardio at 52-y/o is outstanding because of this; I have a resting H/R in the mid-40's and a max H/R in the low 190's and I can sustain a HR of over 160 for a very long time.
However, because of this I'm prone to over-training and have done things to force myself to ride slower more often. But, lately I've failed and now I think I'm feeling the effects of burning myself out -- I just can't stop riding faster.
I just saw this article, hopefully it slows me down
The Art of Riding Slow : Keep Your Heart Rate Low | Bicycling
I once went for a training ride with a professional cyclist—and it was one of the slowest rides I’ve done all year. Now don’t me wrong, Optum Pro Cycling p/b Kelly Benefit Strategies rider Scott Zwizanski is by no means a slow rider. But as a professional who logs thousands of fast miles over the course of a season, Scott understands and appreciates the benefits of an easy ride. “Riding slow gets the blood flowing, while keeping my heart rate low,” Zwizanski says. “It’s an important component of the recovery period between hard rides or races.”