I thought being ambitious was synonymous with having a known hard work ethic, or relentlessly biting off more than you could chew. I'm learning working small and consistent is ambitious, too.
Ambitions don't need to be externally overbearing, working like a bull-dozer to force achievements. Implicit determination and discipline is ambitious too.
Pick something, want it enough, and go for it. Tell people if you're compelled to. But, have confidence that even if you don't tell any one, your ambitions are just as real and meaningful.