Has it been too long to start a relationship?

If you like her being your friend, stay that way. Don't force the issue and if you ever do pursue her as a girlfriend, make sure that you are both on the same page. In high school I was in a relationship with my best friend for 3 weeks and then the relationship ended and so did the friendship. It was not worth it at all. Just think things over a lot before you get in over your head. Don't screw up something awesome for a fling. If she is truly your best friend, you should be able to talk to her openly bout this, but that doesn't mean you should. Just take it easy and if the time comes later on, it will come.