Is there really? I'd be interested in what you mean by that, and what you think that difference consists of in that context.

There are different kinds of fitness, of course, and the relationship between fitness and health is almost certainly not linear. There's absolutely no doubt that riding for extended periods at low intensities builds aerobic fitness, improves fat metabolisatipn, and, therefore, endurance and the ability to maintain a good pace for increasingly long periods. Most people would call that "building fitness", I think, and there's solid evidence that extensive moderate exercise, such as low-intensity cycling, makes you healthier. All of that is available to the new cyclist without ever getting into intense workouts, and that's what I was referring to when I said it should be encouraging.

If you want to get fit in the sense of being fast, you need to work on threshold power and VO2 max. Unlike aerobic efficiency, that can't be pushed up from below very effectively, it has to be pulled up from above. That's where the high intensity comes in. But it remains interesting, to me, that even elite cross-country skiers and olympic rowers (two of the groups that tend to record higher VO2 maxes than almost anyone else) train at high intensity only about ten percent of the time.

Both types of training, as far as I can see from my reading, contribute to both building and maintaining fitness. But they do so in different ways, one training the aerobic systems and one the anaerobic.

Am I missing your point?