The ratio makes sense though it almost seems low. The way I see indoor riding, especially spinning class that can be rough, is that almost 100% of the time is useful for training. When riding outdoors, there are often flat "walking pace" times, at least with me. In an indoor class, the instructor forces pace and resistance and it really is difficult. I guess it'd be the difference between sitting on an exercise bike but being unstructured and having an actual training plan.

Then again, maybe training is training and I'm confusing muscle training with cardio benefits.

Ack, I think I'm confusing myself!