A couple of "pro" points here:
You say you've "had feelings for" your gal-pal for some years, and that you've lately begun to see her as a possible girlfriend. If so, you've already lost her as a friend-- not in her mind, but in your own. She can easily go through the rest of her life thinking of you as a friend and acting toward you as a friend, but you will never be able to think of her or act toward her that way again. To you, she's no longer going to be Jackie or Sheila (or whomever) she's always going to be "What If?" You'll never be able to see her again without thinking "What if I had asked her?...What if we'd just tried...?" The only way you can get her back into your mind as "friend" is to put her there as "girlfriend."
Besides, I think you might be exaggerating the likelihood that your relationship with this girl will go South on you. Relationships between chance-met people most often end because people learn something about their partner that wasn't obvious when they met: "Oh My God, he's on the FBI's Most Wanted List!"...or..."How in Hell did she get in that video, and who are all those guys in it with her?"
This is a possibility you've already avoided with your long-time friend. I'm sure that after 12 years, you know all there is to know about her background, her personality, her character and habits-- every place in her existence where deal-killing revelations might hide; and she knows all the same about you. Since you've stayed friends for more than a decade you're obviously both comfortable with what you know about each other, which means you've already done 90% of the work needed for a successful relationship.
I say "go for it."