I am a real convert, and it basically came about (so to speak) by listening to a couple of slightly older women I know raving about it.
I'd always been a bit shy of the topic, and had always enjoyed vaginal intercourse so much that it never really occurred to me to try it. However, once my curiousity had been spiked, I found that I was always trying to read about it, and I even started hunting down pictures on the 'net. Needless to say, by the time my man and I finally did it, I was absolutely craving it.
It definitely has a kind of dirty illicit thrill to it, and I certainly don't find it uncomfortable or painful - I really look forward to the "full" sensation.

Perhaps you could get your lady to read a few of the following articles. They will lessen any fear she may have, which is often simply fear of the unknown.

As a final point, a very covert informal poll of the girls at work would tend to indicate that women over 30 are more inclined to add it to their repertoire. Another reason to favour more mature women.....

www.dizzydee.net/tutor/anal/anal.htm
www.minou.com/aboutsex/analsex.htm
www.the-clitoris.com/n_html/n_anal.htm
www.sexuality.org/l/incoming/analrule.html