Many thanks for your suggestion, and support for TrainingPeaks Virtual.

I'm delighted to say that there are many ways you can do this. As other commenters have noted, there's already the coastal loop (and several variations of it) that provides a variety of long flat routes. In addition, you can now upload any GPX file you want and just ride that - so you have the ability to ride any route, anywhere in the world (either real, or imagined), so you can have something as long, and as flat as you like!