Using the Phil Maffetone approach of training at a HR = 180 - age +/- some adjustments for fitness and injuries. There is good information about being a healthy athlete there.
In practice I found that 60% rides worked. I used them extensively last November and December to build a base after minimal riding and training in 2018. Doing just 30-60 minutes is humbling when you are coming back, As my fitness has improved so has my MAF capacity: e.g., Perkins-1. Log In to TrainerRoad