One option is to create a separate smart playlist for these lectures in iTunes similar to the one titled Recently Played (or Last Played in your case. To create a new smart playlist, choose File -> New Smart Playlist.
Make sure the Match the Following Rule is ticked.
Rule 1 should be Last Played in the last X number of days, weeks months. X can be any number of days weeks or months you see fit.
Add another rule that is set to Playlist is <name of your newly created playlist for the lectures>. So you'll need to create a separate simple playlist (File -> New Playlist) and add in all of the lecture files.
Then alter the Limt to X items selected by most recently played. Again, X can be any of number of items you see fit.
Leave the Match only checked items setting unchecked.
Lastly, be sure that the Live Updating option is also ticked. Then click OK.
Plug in your iPod and configure the new Smart Playlist to sync to your iPod from under the iPod's Music configuration pane. Or if the iPod is set to Sync Entire library, it should automatically sync over the next time you connect your iPod Classic.
There are several other ways you probably could have configured this Smart Playlist that are better than the one outlined above. So if you are up for the challenge, I encourage you to try different settings with the smart playlist and see what happens.