Trimming podcast

You've been a little unclear here, I'm afraid - let's run through all of the possible options: Are you talking about a session in multitrack view, or just a single file with a load of silence at the end? If it's the latter, then simply highlight the silence and hit 'delete'. If it's the former, then that's not real 'silence', it's just a space at the end of the session where you could add more clips - clips are bits of files that you include in the session, and there are generally lots of them. If you have a single clip that you don't want to use all of, then grab the trim handle at the end of it and drag it back to where you want it to stop.

If it's a session, you highlight just the clips you've used in the session by swiping the time on an unused track (you can then trim this selection on the timeline to make it accurate), then select  Multitrack>Mixdown Session to New File>Time Selection, you can have your multitrack session as a single file that will run for exactly as long as the selection you made. And it's a single file, which is much more reliable to play out.