You've got some good advice here. I think if I were in your position, even after having been poly for a million years
I would take a good long time to enter in to something more than friendship. Friendship and partnership "feel" different. I think I would be very cautious about taking on that feeling too soon. I would want everyone to be good and used to the feeling before changing my friendship into something closer and sexual.
It sounds like you don't think you owe her anything. I think that is a blessing. I would be concerned that I owe her an answer of some kind. Even if I know I don't. Good for you.
I also suggest you do a lot of educating yourself on here and elsewhere as to what could come up. Its all very well to plan, but really, you'll never really know what will happen until it happens. Better to be prepared I think.