I was forturnate when I banged my head. I didn't suffer any lingering pain because the helmet took most of the blow and entry frame is very well padded. Nonetheless, it scared the crap out of me because it happened so fast. It was a big wake-up call for me. Up until that point I was pretty much fearless with respect to trying new things. My coach also gave me the "don't do that crap again" look, and I got the bail-to-your-back speech.
I would rate my back skills as decent, but I'm biased. I'm not very smooth but I can go any direction I need to. My coach told me we'd work on some more bail-to-back exercises next time I go to the tunnel.
I was hoping there was some sort of exercise that would help with the endurance issue. Perhaps, as you said, I just need more tunnel time. I was bummed last time I tried to sit-fly because I was finally starting to feel what "stable" felt like, but my arms/lats were screaming at me so I had quit for the day.