Some max vo2 intervals, some LT intervals and increasing the amount of time and long rides at aerobic threshold which is rather low HR. But that has to be spaced out with significant recovery either off the bike or very low HR rides usually on a trainer to control the HR. "Why"-just finished a single pace line 52 mile ride through rollers with wind at 19 mph. I rode close to or at the front much of the ride and took some good pulls . It was 80 degrees when we started and 90 when we finished. If I didn't have a "complex" regiment I couldn't do that at age 69 about to be 70. The other riders are a lot younger than me some of them my sons age. Use to train to race now I train to ride with reasonably strong recreational riders-not fair but that's life.