I think of myself as a BIG fan of romantic relationship that evolves out of deep friendship and yours would be no exception. That being said, there is a risk to crossing over from fellowship to romantic couple. For one thing, your fiance may not require the friendship to modify. She may love you like a chum, but not have feelings that run deeper. Another potential problem with crossing over, is that if the romantic relationship end, the friendship also ends as the pain of losing the romance cannot bear going back to how things used to be between the 2 of you. Eventually, some couples make great buddies and pathetic lovers.
On the flipside, there are lots of advantages of moving a friendship to a romantic relationship. So, how does one know if it's time to move from comradeship to couple? The best is that she would have taken the chance and shared with you her level of interest in doing so.
A love worth experiencing is a love worth a risk. To me this means taking a leap into the dark and get out of the friend zone. You could plan a trip for the two of you, which would give you the advantage of indicating how well you both get along and how crucial she's in your life. A trip would give you the added advantage, should a romance be on the horizon, of making a chance to become more than friends and move the friendship to intimacy.
If arranging a trip is a little overwhelming, make it a short break. During dinner, you can recap the day's events and point out how well the 2 of you get along and how much you have in common. As she agrees to your points and observations, you can take the biggest risk and tell her how much you appreciate her in your life, how you've come to realize she is the right girl, and you want to pursue these sensations. Her reply will tell you all you need to understand in order to decide if the friendship is ready to move to a higher level.