Circuit Experience alone will get players to Level 20, and takes way less than 10-20 hours. Source:
our easy money guide.
Then again, from the same site, only 16% of players have beat 60% or more of the campaign. Another interesting angle: according to the site slightly over half of the players haven't even driven over 500km. That's a lot less than 10 hours (of racing, anyway)...
There's a lot of mysteries with the stats. If the game is pulling in roughly 600k players every week, and roughly 100k are Sport Mode players (and thus, the regulars), what are the other half-mil doing? They're not really progressing through the game it seems. And new players only account for a tenth or less of that.
Maybe people are just practicing in online lobbies? No XP/Credit gains that way. Or they're painting/Scaping...