I fully agree the explanation of jtupper-ware, that can be very much better of mine because the knowledge of exact English terminology (for me is more difficult to explain).
I published as IAAF Academy a work about the use in training of special circuits for increasing strength endurance, in order to create the MAX LASS (maximum lactate steady state). This is not a research, but consists in the evaluation (only using lactate levels) of some of the best runners (including the WR holder of steeple Saaeed Shaheen) in different part of the season, after a test connected with their training and their specific event. We saw that, when their "specific" shape was growing, they had the ability to stay at almost the same level of lactate (very much higher than the classic threshold) for a long period of time, depending on their type of training. For athletes in top shape for 5000m (duration under 13') we can suppose an ability to stay about 9-10 mml for the central part of the race (for example, during second, third and fourth km), only moving from 8 to 10 mml, while the last km can launch their lactate till values of 14-16 mml.
If you take the lactate immediately after the test, you cant have a real value, because for the next minutes the concentration of lactate in the blood continues to grow, while the production of lactate inside the muscle fibres already is over.
Going to the marathon, the new type of athletes have a high ability in buffering. One of the reasons of this may be the improvement in the permeability of the membrane. One of the ways for defining "Endurance" is "the speed of removing lactate from muscle fibers or buffering".
So, if with marathon runners "old style" our goal was to reduce the production of lactate at the speed of the race, in order to teach to the muscle fibres to use as fuel, at the same speed, more fatty acid and less glycogen (and this fact could build the ability in lasting longer at the Marathon Pace), now we look for increasing the speed of buffering, allowing the athletes in using a part of the lactate they produce.
A top marathon runner till 2000/2005 was no more able to overtake 10mml with any type of test, because the goal of his training was to reduce the produiction of lactate (that is connected with the consumption of glycogen).
A top marathon runner of today is still able to reach 15mml of lactate, if we do a max test (for example, 2' uphill at max speed).
So, they can use a percentage of lactate (may be 20%) as new source of energy, and this fact means not only that the average of speed for all the marathon can be faster than before, but also that the index of resistant between different distances (for example, HM and Marathon) has the trend to become more close. If till now we spoke about a 5% of difference between the HM speed and the Marathon speed for a well trained athlete (5% of 1 hour in HM is 1:03:00 that is a marathon in 2:06:00), now we look for reducing this index to 4% (in the above case, 4% of 1 hour is 2:24, so 62:24 x 2 = 2:04:48).
If we are able to create for an athlete able running 59' in HM (and now there are many) this level of specific endurance, we can have somebody able running in 2:02 in short time.