This may not be directly related, but I have several episodes of the Church of the SubGenius "Hour of Slack" on my iPod. The episodes I have are encoded as 80kbps VBR Ogg Vorbis files. Each episode lasts about 57-62 minutes, and I can listen to about 8 full episodes on one full charge, before my iPod is fully drained part way through a ninth episode.
If it's purely speech you're listening to, you can either try re-encoding in a lower bitrate (96kbps works well for speech only), or do what you suggested and split the files up. To me, though, one long file seems to last longer than splitting files, since the hard drive would have to spin up more often to load each individual file.
You could try looking into the Speex codec for the language tracks.