How do I change the playback speed in iTunes?
This is not natively possible in the Mac version of iTunes (nor Windows for the matter). You don’t mention if you have OS X or Windows. If you have the former, you have two choices: 

1) If the podcast is an MP3 file, you can convert the file to an AAC file in iTunes (with an extension of m4a.) and then change the extension to m4b; the file becomes an audiobook which can be played faster if I am not mistaken.

2) Another alternative is to hear it in Quicktime, which has speed controls: Right click the file in iTunes and choose "Show In Finder”. Option-click the file (right click) and select “Open With…”, select QuickTime. Once in Quicktime option-click the "Accelerate" button to change the playback speed.