Be careful.
I was in a similar situation with a girl who I knew really well and had a huge crush on, and we hung out together loads, but she didn't like me in that way, and it turned out badly. We get on well still, but she is very cautious and it's not the same.
Someone else when I spoke of this matter before it all went wrong said that I was "too far up the wrong ladder" in terms of the relationship we had.
This may not apply to you, as she may in fact actually like you back that way, but be careful about it is all I'm saying, you don't want to break so many years of friendship.