I'm glad we didn't chase you off!
My current best friend (my life's been in flux, I moved back to Florida 3 years ago, then got divorced over the last 18 months, so when I refer to her as an old friend I'm talking about a little more than a year) is a friend-zoned fellow infj. And I'm damn glad to have her as a friend for many reasons. This is the wisdom I've learned from the friendship:
1. If you have feelings, ask her out. If she rejects you and runs away, so be it. Get the feelings on the table. I asked this person out last year and was rejected, maybe for external reasons (I was still married at the time albeit completely and irreconcilably separated and having filed for divorce), maybe for other reasons of her own that she's chosen not to share with me. It's her business, I've chosen not to ask her about it since. Believe me, it wasn't easy to do, I stuttered when I asked, and while she went out of her way to be kind in her rejection, it wasn't fun internally for me. But she knows how I feel about her for disclaimer or for future purposes, I'd feel dishonest or ambivalent otherwise.
2. Honestly assess whether the relationship is reciprocal. If you're just chasing these people, probably not the best friendship. This friend of mine reaches out to me, invites me along to things, all of that. Yeah, I'm a subservient little f'er, so I'm willing to go extra miles, but I can say that it's worth it for what I get in return. Sure, she uses me to complain about her bf. But, she's there to lend an ear when I complain about my (now-ex) gf. And the stories, fears, and advice we share are I hope useful to each other now and in the future. Everyone needs and deserves to have that.
3. Keep a pulse on your own feelings. The best measure here is when you see this person having a good relationship with a partner, who do you feel jealous over. At the point where I could feel jealous for her, that she was in (at least at the time) a good relationship with someone, and not jealous for the guy she was with, that was when I could rest a little easy, despite the fact that I do have some idle daydreams to the contrary.
4. (Similar to 2), assess your needs in life and where this/these friends fit. This woman I'm talking about fits very well in my life. I hope she'll be there for a long time, we have mutual friends and interests, I rely on her a bit for spiritual and relationship advice. And yeah, I find that especially when I don't have any significant someone, it's nice to have people to smile at, flirt a bit with, etc. It makes life less lonely, and it leaves me in a better place and a stronger position when I think about whether I want a relationship, with whom, and how soon.