The excellent book “Fast After 50” by Joe Friel has a solid research background. He firmly believes that periods of intensity are required for us aging athletes. He caveats that by saying it needs to be a limited amount of intensity followed by true recovery. As we age, the recovery process slows, so that you can’t do a double century on the weekend, followed by a fast group ride on Monday, hill repeats Tuesday, etc. Probably limited to only a couple intense sessions a week with easy rides in between. Additionally, as others mentioned, he talks about strength training to help avoid injuries while doing your intense workouts.