Strong feelings for someone in a relationship

So about nine months ago I started talking to one of my co-workers, eight of those months we only talked at work, and sometimes we'd chat on Facebook. Recently, specifically three weeks ago she told me she liked me. That's all great and made me very happy because the feeling was mutual, but the problem is she is in a long distance relationship with a guy that lives a few states away and is in the service. They've been together six months and only see each other a few days every couple months.

Anyways, ever since she told me she liked me we've been hanging out more and more, and we're always texting. About 4 days ago I told her in more depth how I felt about her, and that I'd always be there if she needed someone. She responded by saying she'd be with me if she wasn't with him, and that I was such a great friend.

I guess I'm just not sure what to do because everyday for the past few weeks when I wasn't with her I've been completely down and depressed. Let me also point out that I've never had a girlfriend my entire life and I'm 20 years old. Thus far I've held back any physical emotions with her. I would never sink to that level, because I know she is in a relationship. Either way, I don't mind being her friend but I'm only human and I really want more. I've never felt like this about someone before, and I'm really clueless as to what my next move should be.

Do I wait it out and be her friend until or if something happens?

Do I distance myself?

I just don't know... And I really don't want to let go because I really do care for her.