Explanation needed of a quote from the movie 'The Notebook'...?

When Ally said to Noah after they had sex the first time "All this time, that's what I've been missin? Let's do it again", what did she really mean? Mainly the 'that's what I've been missin' part. Did she mean that as to say, is that it, like he didn't last long enough or did she mean it as to say that it was so good she can't believe she missed out on doing it for so long. I'm confused.