A shed
I totally disagree with you.
When I went in 2005 with a mate of mine, Id done about 50 real life laps at that point, he'd never been before but had logged 2000+ on the playstation.
He knew the lines FAR better than me.
Dont get me wrong, you need to still get a few laps in before you go hell for leather as you need to feel the gradient changes etc which make the car go light in ways you dont notice on the game, but in terms of learning the lines its an epically useful tool!
My first Ring lap i was like 'OMG this is nothing like Forza!!' but on the second lap once the overall shock had worn off it all came back and i felt i knew what corners were coming really well thanks to Forza - its a good tool for learning 'left, right, long right, left' etc and then build on that foundation in person