Thanks for the writeup.  I am a firm believer in interval training, but you are absolutely right that it can't totally replace the long distance or tempo runs.  To me, the intervals are nice from a mental perspective: time seems to go by fast.

The ultimate in short interval workouts is the tabata workout: 8 x 20 second intervals with a 10 second rest.  You'd have to Google "tabata" for specifics and to get the science behind it, but I can say that they are quick but extremely tough.  Doing them on a spin bike is the toughest, but squats or burpees are good too.  I've never tried it while swimming...