Actually, I think the jury is still out on whether or not Robin was real. Okay, the legend isn't 100% factual, but that's not to say it wasn't based on a real person. But regardless of whether he's "real" or not, there's a strong and gorgeous tradition of MCs meeting legendary figures. The Kipling collections, "Puck of Pook's Hill" and "Rewards and Fairies" leapt to my mind immeditely, because I used to read them to my babes, but there are very recent, successful examples as well - just ask your librarian!
The main thing I wanted to say though, was this: You're going to meet
so many people during the course of your life as a writer who are going to tell you your ideas are lame or have already been written or aren't worth trying, and you'll have to find a way eventually to stop hearing them - so start now

. Really. You have an imagination, and you're a writer - they don't get to shut that down, however much they might feel bothered by it.
Most importantly, you'll have to learn to differentiate between that kind of ignorant, unhelpful criticism, and the proper, useful kind. Mostly, if they're talkling about a book
they haven't read because
you haven't written it yet, you can just ignore them. If they're talking about grammar or plot or characters in a book you
have written - well, that's a whole other story.
Good luck to you - keep imagining, and
keep writing!