I loved this video by John Brown:
At about 3 minutes in, he goes into super-fast coding mode. He must have recording himself working for a while. Then, played the video back at a higher speed and recorded himself talking of that. It makes it a bit more exciting! You can certainly cover a lot more ground that way, as well as free your mind up a bit. When your coding, you're just coding. When you're explaining, you're just explaining.
Here's another one:
I typically use Screenflow for video screencasts. I know that once you have a video track in there, you can adjust the speed by adjusting the duration.
Then you could watch that while recording a new audio track, which you can easily do through Quicktime Player (File > New Audio Recording). Quicktime Player does a fine job with audio recordings - that's how we do it for ShopTalk. Then once you have that audio file, you could drag it right back into Screenflow.
I'm sure there is a bunch of ways to do this, I just wanted to make sure to cover at least one. Another way might be to automatically snap a screenshot every few seconds as you work. Click can do that, or I'm sure there are command line ways to do it. Then turn all those images into a movie. Again likely lots of ways to do that. The old Quicktime could. That's how this video was made.