I worked on three different startup ideas unsuccessfully before moving into an existing business. I learned a little in the unsuccessful startups but not a great deal - it's hard to learn when you don't have a great amount of traction / users etc.
When I moved into the existing business suddenly I learned how to sell, how to work with customers, how to position the brand etc.
I wish I'd sucked it up and taken a more junior role sooner - I think I would be further along by now.
I think the idea of getting real exposure sooner is underrated.
Incidentally, this is why your Startup Apprentice role is so interesting nedwin ;).