@mprevost @KIWI5 poses an interesting question as well... what are your thoughts on this.
Thanks,
This is from my run guide (at mikeprevost.com). It is related to the question. The answer has to do with steady state. Once you hit steady state, your physiology is relatively constant.
Going Long: why it is ok to break up your long run
When we start to run it takes time for your heart rate, respiration, and metabolism to gear up. If you use a heart rate monitor you see this every time you training. The same thing happens when you shift gears while training. If you are doing a tempo session, for example 5 minutes easy - 10 minutes tempo, it will take 1-2 minutes for your heart rate to settle at the new intensity when you make the shift from "easy" to "tempo.". This is called reaching steady state at the new intensity. It turns out that at some run intensities you can reach a steady state, where lactate no longer increases and heart rate remains relatively stable (although a small amount of heart rate drift occurs). Let's take a look at a concept called maximal lactate steady state (MLSS) to explain this concept in more detail.
The graph shows notional data during a typical maximal lactate steady state test. This is representative of the kind of data I have seen in the lab dozens of times. A typical test would consist of several stages. Each stage lasts 30 minutes and we do only one stage per testing session. We have the athlete run at a steady pace and measure blood lactate levels every 3 minutes. We start at 4 miles per hour, and increase by 1 mile per hour each session. We might get a graph like the one above. What you can see from this graph is that at each speed the subject reaches a steady state, although at higher and higher lactate levels, until 9 miles per hour. At 9 miles per hour, the subject cannot maintain a steady state and lactate continues to climb, despite the speed staying the same throughout the test. However, at 8 miles per hour, the subject reached a steady state, with lactate stabilizing around 3 mmol/l, even after 30 minutes of running. In this case, the subjects maximal lactate steady state (MLSS) occurred at 8 miles per hour. For most people, MLSS pace is sustainable anywhere from 30 minutes to one hour. If you look at a person's physiology (heart rate, lactate, respiration etc..) at 5 minutes, 10 minutes, or 25 minutes into a run at MLSS, there is very little difference. Think about what this means in terms of splitting a run that is slower than MLSS. It takes 3-4 minutes to hit steady state. So 5 minutes into a run or 40 minutes into a run, physiology is going to look very similar. So what is the difference between 2 X 10 minutes at MLSS or simply 20 minutes straight at MLSS pace? Not much. Because physiologically, most of it is spent at steady state in both cases.
MLSS is a zone 4 effort. Long runs are typically run at a zone 2 effort (
refer to the training zones chart shown earlier). Zone 2 is well below MLSS. So the entire run is going to be run in a relatively steady state. For our run subject in the chart above, this might be at 5-6 miles per hour. Heart rate would increase very little during the run and lactate would be steady at approximately 2 mmol/l. Your physiology is not going to be much different 5 minutes into a zone 2 effort run vs 1 hour into the run. So, what is the difference between doing a 2 hour run vs 1 hour run in the morning and 1 hour run in the evening of you are running in zone 2? Again, not much.
This also explains why during Base of a periodized run program that it does not matter much how you structure your runs. 3 runs of 10 miles have about the same fitness impact of 5 runs of 6 miles when all of your running is zone 2.
More and more ultra-runners and marathoners are splitting their long runs and are finding that they recover better and fitness is not negatively impacted. For example, rather than doing a 30 mile run on Saturday, an ultra-runner might do 10 on Saturday and 20 on Sunday. Or 10 on Saturday, 10 on Sunday morning and 10 on Sunday evening. In each case, the runner accumulated 30 miles of running on the weekend. The fitness building impact of those 3 different options is very similar.
Before we completely throw out long runs, it is important to state that long runs do have their place. They are important for mental toughness training, for trying out race strategies, and to build confidence and check pacing strategies. However, because of what we discussed so far, you don't need many of them.