Cougar – Member
Back in my single days, I've had a couple of friends who went from "friend" to "friend with extras" and back to "friend" again. I'd say it depends entirely on the friend, it's very difficult to predict what will happen.
One thing I would say though, is to be very clear up front as to whether you're proposing "hey, let's have a spot of horizontal jogging" or "you know, I've always loved you, shall we take our relationship to the next level." If she's expecting one and getting the other, it could be messy.
(Though if it's not messy, you're not doing it right)
+1