The biggest advance in running training has been the gradual acceptance of the idea that one can train for various components of a race. It started with the coaches of sprinters working on form and has slowly spread to longer distances. There are a multitude of ways that this has been done and I'm going to discuss two ways which can be compared to recent (i.e. in print) training manuals, one for track and road and one for trails.
Though the science is very iffy, races can be broken into time segments based on physiological needs. Races of 5-15 seconds are completely dependent upon ATP and creatine phosphate levels and one can spend that amount of time in longer races running in that way, which is generally just called sprint training. Races of 30 seconds to 3 minutes burn glucose anaerobically, producing lactic acid and training for this is generally called lactic acid tolerance training (in Jack Daniels parlance, this is "R" training). One can run a further 2-10 minutes at near maximal respiration and heart rate, called VO2max training ("I" in Daniels). Beyond this, one can run up to another hour at a rate that is not completely aerobic, called anaerobic threshold ("T" in Daniels) and 2-6 hours at a completely aerobic level ("E" in Daniels).
To prioritize your needs for a race, take the goal time you determined in Step 2 of this series, and subtract the longest time in each range above, from fastest to slowest. For example, a 3:00 marathon would be 1/4 minute sprint, 3 minutes lactate, 10 minutes VO2max, 60 minutes anaerobic threshold, and about 107 minutes aerobic. That means almost two thirds of one's time is aerobic and one third anaerobic threshold, so these should be what one's training should focus on, with just occasional runs working on the faster levels and completely ignoring the sprints.
Similarly, a world-class male 1500 meter runner would focus only on lactate and sprints, a world-class (13 minute) 5K runner only on VO2max and lactate. For middle of the pack runners training at about 10 minutes per mile, that would be 1000 meters rather than 1500 and 3K rather than 5K.
The strange world of trail 100 milers
None of the above means anything if one plans to run, say, 100 miles on a trail in 35 hours. All the levels listed above only add to seven hours, leaving 80% of one's time unaccounted for. Thus, an ultrarunner's training should be mostly devoted to paces slower than the aerobic level, somewhere between a slow recovery run and walking. The first big question I had to face when I decided to train for such a race was "How do you prepare for running all day if you have no idea how fast you can run all day?"
The answer seems to be (and it came to me long after the fact) to do two different types of long run. One type of long run is meant to be faster and "burn through" the faster aerobic and anaerobic levels, leaving one forced to slow to a shuffle, which seems to be possible to run almost indefinitely; this "shuffle pace" should be noted, as it's the pace of the other long run. The second long run is the ultrarunner's bread-and-butter, the extremely long run (4-7 hours or occasional low-key races even longer), done to get used to running slowly but consistently under any circumstance, testing out equipment and foods on the way, to see what works.
The race can still be divided into components, however. These races are invariably on hilly technical trails and much of the race has to be run in the dark. Therefore, the components are: the long run, the hill run (hill repeats), the technical run (difficult trail) and the night run. Having a more flexible schedule than most, I started my long runs later in the day, not realizing that everyone else was getting in hours of specific training in the dark before I started. Because I can run for 5-6 hours comfortably without eating, I did not get used to being able to run while digesting, which is crucial when running for an entire day. I ran in places where I could get water regularly, so I didn't train carrying the weight of 70 ounces of water on my back - giving me back problems in races when I suddenly was forced to carry things.
It's not surprising that I did not do well!