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Deciphering the Long Slow Distance Run (Part II)

Posted Apr 23 2011 11:23am
photo by Ken Lund
In the previous post, we discussed the basics of what a long run is and why it is important, at least on a practical level.  Here, in this post, i’d like to expand on these concepts, talk about speed as it pertains to the long run and discuss why it is so important to run long runs at the appropriate pace to reap the most physiologic benefits.

So what is the appropriate pace and speed for LSD? How fast should I do my long run?
If you’ve been running for a while and have several races under your belt, then you can figure out your appropriate race for long runs by plugging your race times into a race calculator (i.e. McMillan or Daniels ) and reading off the number.  If you are new to the sport or have no recent races to use for this purpose, then I suggest you run at a comfortable speed that would allow you to maintain a full conversation if you were running with a friend.  If you find yourself having to take a breath at the end of every sentence or every few words, then you are running too fast.  By whichever method you choose to figure it out, you will soon realize that the appropriate LSD pace is indeed a manageably slow pace and sooner or later you will be tempted to run them at a quicker pace than is specified.  If you’re just starting out or are pushing mileage above what you’ve been running, I implore you to resist that temptation and consciously slow things down to the appropriate pace and effort.

So why is it important to run the long runs slow? Doesn’t running slow teach your body to race slow?
Believe it or not, that is the most frequently asked question I get on the subject. It is also the biggest mistake that most beginners make when training for their first (or second) half or full marathon, which is that they simply run their long runs too fast.  To understand the danger of running your LSDs at too quick a pace, you have to know a little bit about the metabolic mechanisms by which the body generate energy for long distance runs.  Stored energy (as it pertains to running) comes in two forms – glycogen and fat.  Glycogen can be rapidly broken down to simple sugar (glucose) to be used as a readily available fuel source.  Fat can be broken down slowly to triglycerides and fatty acids and yield a more powerful and longer lasting supply of energy.  The difference between these two main fuel sources is that while glycogen can be quickly converted to usable energy, their stores in muscle and liver are somewhat limited.  Fat stores on the other hand are essentially limitless and yield more energy per molecule than glycogen, but they also require more oxygen to be converted to energy and therefore can only be utilized in a strictly aerobic (with air) environment.  In practical running terms, glycogen is the main source of fuel for running fast and short because its supply is limited and rate at which it’s consumed is fast.  Fats are used to fuel longer and slower runs because its supply is not limited and rate at which it’s consumed is slow.  For long distance running and racing then, it is easy to see why utilizing fat stores as the main energy fuel source would be advantageous.  However, by the same token, it is important to realize that one must run slow enough to provide enough oxygen to the working muscle to allow for local fat breakdown.  If  the speed is increased and the oxygen demand rises above what the lungs, heart and blood can deliver to the working muscles, then fat breakdown stops (due to anaerobic conditions), and glycogen breakdown must take over.  However, because liver/muscle glycogen stores are limited, they will soon run out and cause the runner to bonk or “hit the wall”.  As you can imagine even if you’ve never been there or felt it before, it’s not at all a pleasant experience.  The whole purpose of long run training then is to train the body to burn fat for fuel and avoid glycogen depletion as much as possible. 

For the performance seeker, running LSD at slower pace is important because it’s the most effective way to build a strong aerobic base upon which faster and more race-specific training can be built upon.  What do I mean by that?  Well, imagine the runner as a race car with a good engine but a limited fuel tank.  If that runner only does fast workouts, he or she is teaching the body to only utilize the short-term energy source to run/race.  In that scenario, although there may be some gains in speed and efficiency from training, success will always be limited by the short supply of stored energy.  To use the race car analogy that would be similar to having the best engine and body parts but not having a big enough gas tank to take advantage of it.  That runner may have the capacity to go very fast, it just can’t go very far and will falter in a race of any distance.  On the other hand, if the runner learns to incorporate slow long distance runs into the training regimen, he/she will teach the body to use the slower-burning and longer-lasting alternative to glycogen, namely fatty acids/triglycerides, as the primary fuel source.  That will enable the runner to run for longer, building stamina and endurance in the process.  Essentially then, running LSDs (at a slower pace) builds a bigger gas tank for your engine!  The caveat again though is that you must be careful to do these slow enough to only engage your fats as the primary fuel source  and not tap into your glycogen reserves.  Remember that the process by which fats are converted to energy (called fatty acid oxidation if you want to be scientific about it) require oxygen first and foremost, and the only way to provide that is by making sure the effort is easy and confortable.  Especially if you are just starting out and/or building mileage, run your long runs slow and resist the urge to speed up.  If you want to run faster, train to speed up during your track workouts/tempo runs…not your LSDs.  Your body will reward you not only by making the faster efforts feel easier but by resisting injuries too.  As my running partner used to say:  (As long as you know how to run), no one ever gets hurt by running too slow!

What about fast-finish long runs, marathon paced runs or progression long runs?  Where do they fit into the context of long distance racing and training?
In a way, long runs where you’re looking to run the last few miles fast (fast-finish), some middle miles fast (marathon pace runs), or each mile faster than the previous (progression) aren’t really LSDs because they serve a different purpose than building endurance and stamina.  As a result, I would advise against incorporating these into your training until you are comfortable running that entire distance slow.  For example, if this is your first time tackling a 16, 18, or 20 miler, the primary objective wouldn’t be to run any portion of it at faster than conversational pace.  It would defeat the purpose of LSD as stated above since it would increase oxygen demand and tap into glycogen stores.  Having said that, once you are comfortable with the time and distance of the LSD, it would not be a bad idea to incorporate some faster miles into the long runs.  Understand though that the benefits of these runs are probably more psychological than physiological and the risk of injury would definite increase the faster and longer you go.  I personally learned that the hard way a month or two ago when I ran a 13 mile marathon pace run on a hilly terrain, and strained my hammy and calf.  It forced me to scale back my training and stressed me out for a full two weeks!  It was definitely not fun and not recommended!

Okay, so that concludes my discussion on LSDs and long runs.  I hope I was able to answer some questions you may have had about them as it pertains to distance running and training.  Furthermore, I hope I was able to convince you that easy and slow is the way to go when you’re running long, at least in the beginning.  If there are any additional questions or concerns that I failed to cover, leave a comment or contact me and I’ll see what I can do to address them. 

Good luck to all of you in your running, training, and racing!

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