I had no true idea how much there would be to learn.
When I first dabbled with the idea of learning to fly, I knew there would be learning involved. I knew you had to learn about airport operations and how to communicate with ATC. I didn't necessarily understand how much there was to learn about airplane physics (like left-turning tendencies and such). I definitely did grasp the true importance of understanding weather.
Most of all, I had no idea I'd need to learn how a combustion engine works. I have an electric car and an electric lawnmower. I thought my days with combustion engines were long gone, and yet here I'm learning more about them than ever before. Fascinating.