I think that for the long distance athlete, once you've developed an aerobic base over a few years, it doesn't take a lot to maintain it. The key is to not take a big layoff each year. This way your aerobic engine continues to grow season to season. I make a point of 1 long ride per week with the rest of the time working on speed, intensity or recovery.
As the tabata study proved high intensity will increase aerobic performance. By working at varying intesities and with different interval patterns you can continue to improve as the seaons go by