Hey @83years! Welcome to the TrainerRoad forum!
This is a great question. @alexfthenakis’s response is super helpful!
THIS! So many of our athletes have successfully competed in ultra endurance events without spending too much time in the saddle. Our Training Plans are built around the physiological systems required for specific events. Therefore, you don’t actually have to ride for inordinate amounts of time to be competitive for your ultra endurance distance events and rides. High-intensity interval training isolates specific energy systems to prompt specific physiological responses.
Duration is just one aspect of progressive overload, which we will be sure to utilise as you progress through your Training Plan. However, there are many other ways we can progress your training. In a progressive approach to training, we can’t continually task our bodies with event-distance training. Not only is it unnecessary, it has potential to be a detriment to your training progression in the long-run.
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Your dynamic Progression Levels will help guide how we progress your training! Check out this article here: Progression Levels: What They Are and How to Use Them.
As @alexfthenakis mentions, it can be helpful to do some longer rides, even if just to build your confidence, simply practice being in the saddle for those durations or to practice race nutrition and hydration strategies.
This is where Workout Alternates or using filters to search our Workout Library can be useful!
Let me know if you have any other questions about this 
.