What I learned out there was this- athletes need to do their VO2 work and LT work pretty much year round (that is, if they have a solid base of training).  Why not?  

It seems to us New Englanders that in the base period and build phase period (Fall and Winter), we don't need to do these hard efforts.  I have heard time and time again- it's the "off" season, I just need to do base work (low aerobic training).  However, if you don't consistently challenge your energy systems, how will you improve?  How will your VO2 improve if you don't do any VO2 work?  How will your LT improve?

Yes, VO2 efforts and LT efforts are hard.  However, only 20% of your overall run volume and 30% of your overall bike volume (based on weekly workouts) should be at these hard efforts.  When I explained this to our cycling classes, I got alot of surprised looks.  In general most athletes either don't go hard enough or go too hard too often, resulting in mediocre workouts most of the time, they don't improve, they don't see results and get either overtrained or are sick all of the time.  If you look at your weekly volume in minutes or hours, take 20% of that overall time (for running) and 30% of the overall time (for cycling) and that's the amount of time you can spend in VO2/LT effort.  For swimming it's about 50%.  The rest of the workouts should be dedicated to technique work, etc.  If you are finding that you are doing too much or not enough, change your workouts to follow the above protocols.  You will find that you are getting more out of your workouts, you will be able to go hard when the workout dictates and you will enjoy your recovery and technique work.

Ali