Still haven't managed as much as a metric, though that's in the works this summer, if I can keep this abdominal muscle happy. I think a big part of it is mental. It took me a year to break the 30-mile mental barrier, then another year to break the 40-mile barrier. My brain is weird.
My own issue is that I ride alone, and that gets super boring after a while. I'm also a slowpoke and just having time for rides that are that long--never mind the training--is a little difficult, being a student.