Luckily, you can trim off the beginning or end of any song in iTunes right within the app. Go to a song, Ctrl-click on it (that's a right-click for you Mighty Mousers), and go to Get Info. A dialog box will open, with several choices at the top. You'll want to go to Options, and you'll see a Start Time and Stop Time well where you can specify when the song should start or stop.
It's that easy!
As you can also see, there are controls for the star rating, volume and EQ adjustments, and two handy items involving playback... "Remember playback position" makes any song like an audiobook or podcast, allowing you to jump back to the time you left off when you stopped playing that song. "Skip when shuffling" is good for those old iTrip's with the FM bleeps on them (or any other items you don't want interrupting your workout).
Lots of other wacky tidbits on this Apple page.