Agree with many of the comments above, to be strong over half distance you need to be regularly riding 5-6+ hours total a week in my opinion. Also challenge yourself to ride harder:
Stay in a gear longer before stepping down Maintain a target pace Challenge yourself every week or two around a local 25 mile loop
Above all, riding with others who will push you is a big plus.
The times ive had strong half distance rides have been at the peek on IM training, i.e. lots of road miles.
I do rate spin classes if you work at them, but depends on the class, instructor and how hard you work. A couple of years ago I attended a great series of classes (great class leader), everyone was up for it and were hard sessions, even did a few double spins (90 mins).