Kind of tricky, the whole long distance thing kind of throws a wrench it in.
But lets work it through here:
LTR - Not a viable option, because A) Long distance doesn't work, and B) She just doesn't "do it" for you, and C) you are not a pathetic chode who ties himself down to some girl in a different city when theres plenty of available options near you
FvckBuddy - Possible if she (and you) were down for ONLY sex (no hanging out) and if you guys lived closer. But it's inconvenient to go long distances just for sex. Plus you guys get along as friends too.
Friends - You like having sex with her, and presumably you're both physically attracted to each other, so nope. Only way you go to this is as a last resort if she presses for a LTR. In that case she will (probably) stop sleeping with you unless you agree to exclusivity.
Friends with Benefits - Here we go. The benefits of a LTR without the commitment. You get to sleep with each other, but unlike a FvckBuddy relationship, you can hang out and talk and enjoy each others' presence. While you're obligated to stay in contact with her and call her and text her, you are NOT obligated to EVER have her b!tch at you for being a bad boyfriend, because you're not her damn boyfriend. This is a very good option for a girl that doesn't live close. Custom fvcking Made for your situation.
My ex-gf and I get along very well. We basically broke up only because she moved to a different city. But we still love each other, so every couple months one of us drives to the others' city and stays a couple nights. We hangout, go see the sights, eat out a few times (splitting the bill usually), and nearly always get a hotel room and fvck each other's brains out. Neither of us is under any illusions: we're both free to date other people. We're basically Friends with Benefits (even though it is romantic too). Meanwhile, i get to go out every weekend in my own city and chase tail to my heart's content, and she's free to bang whomever she wants.
You're essentially in this right now. Presumably, she is cool with it at least for the moment. At some point she is going to either A) press for "something more" aka a long term relationship, or B) Want a definition of what you guys are or at least make sure you're on the same page.
Case A) You express that you don't do long distance otherwise it might be an option. You explain that you don't believe two people can have a healthy dating relationship unless they are seeing each other at least a couple times a week. Normally i would say that you might have to break it off if she is not okay with this, however her interest level is pretty high from what i can guess, so she will almost certainly agree to a FWB type of deal.
Case B) You explain that you guys are really cool together and you enjoy hanging out with her a lot. Do not say the words "friends with benefits", but merely explain to her that you enjoy seeing her once in a while and spending time with her, however the logistics prevent you guys from having any thing too serious.
And Do Not Feel Bad. A girl would rather share a high quality man than have a pathetic beta chode all to herself. You are doing her a favor. So long as you are honest and straightforward with her, and make it clear to her that you two aren't exclusive