Over the last twenty years of training I have found in my own training and in looking at the training of others with much better success than I've had that there seem to be four types of workouts you should focus on to build a balanced fitness and run up to the maximum of your potential. These are not the only running you should do but they are sessions that are used regularly in pretty much every program I have looked at that produced unusually good results.
These sessions are not an exhaustive list of all the types of workouts you should do but they are the four main pillars that hold up all of the other training. If you are doing these you will do well. If you aren't you will likely not get the most out of yourself.
Pillar 1- Fundamental tempos- These have many names, I've used Renato Canova's name for them. Jerry Schumacher calls them rythm runs, Joe Vigil calls them, fast endurance runs or 30km tests, George Malley or Bob Hodge would probably call it running a bit faster than normal sometimes. Call them what you want, just make sure you do them. These are longer runs at pace that is faster than training pace but not as fast as tempo pace. For a beginning, low mileage runner this might only be five or six miles long. For a competitive marathoner they might get as long as thirty miles. The key to this session is relaxation at a pace that is not exactly fast but is faster than you do or really could do day in day out for your training runs. A good starting place for these is a minute per mile slower than your 5k race pace. These should be done throughout the training year but you would do them much more often during the early phases and much less as you get into the thick of your racing season. Similarly these are important for all runners but even more so for the developing runner.
Pillar 2- Specific work/max lass workouts- I combine these because if your event is 1500 to 10k then the specific version of the max lass workouts is your key specific workouts but in the marathon the key is much more just specific paced running and max lass becomes much more secondary. For me specific workouts mean running within about 1% of race pace. So if your training to run a ten minute two mile specific pace would be 4:57 to 5:03 per mile. I would hesitate a guess that more effort is wasted on running, supposedly, specific workouts that are too fast than in any other area of training. If your goal is a 10 minute two mile and the quarter repeats you are doing feel too easy at 75 you should NOT run them faster. You should either run them with shorter rest or you should run longer repeats at the same pace. If it is early in the training cycle then you should focus on embracing the efforts feeling easy and try to make them feel as easy as you possibly can.
Max lass workouts are workouts that try to create an ability to hold the amount of lacate, ie acid, in your blood at an essentially steady level while running your specific race pace. For a long time it was thought that you could only really do this at 10 mile to half marathon race pace but it has been found that athletes can produce a defacto max lass at much faster paces, ie 3k to 10k race pace, where they hold lactate levels basically steady for 5 to 25 minutes. Workouts for max lass in the base phase focus on doing explosive work, like short hill sprints, bounding, sprinting etc., while your body has a ton of latic acid in it. This can be accomplished through hill circuits or doing hard track repeats with circuits or hill sprints mixed into the rests between the intervals. As you move to the specific phase these workouts morph to involve repetitions at race pace mixed with explosive sprint reps.
These workouts, both specific marathon workouts and max lass workouts, are only a small change from traditional or "normal" workouts. Do not fool yourself into thinking that you are fine just doing your regular sessions and working hard. These sessions are probably the biggest difference in the performances we see today verses the 1980's. For a long period of time it seemed that the best in the world were sort of pinned into the 13 teens, 27 lows and 2:07 to 2:08 range. Now I'm not trying to say that EPO wasn't involved in the sudden proliferation of sub 13 men in the 1990's. I'm not that naive. That said there were many highly dedicated very hard working and talented men in the 1970's and 80's and only a few slipped under 13:10, notably Henry Rono and David Moorecroft. Even if you discount some of the best in the world today there are many athletes who I feel very secure in saying are clean who are running well under 13:10. Ben True, Hassan Mead, Chris Solinsky, Matt Tegankamp, Sam Chelanga come to mind very quickly. I simply don't believe they are more talented then say Matt Centrowitz Sr., 13:12 or Lasse Viren, 13:16. When you look at this improved standard and then consider that the vast majority of the training is unchanged in the last fourty years it becomes clear that something subtle but important has happened. Both generations did high mileage. Both generations did tempo runs, intervals, long runs. Both generations ran on mondo tracks and even had pace setters, though admittedly Viren never ran well in a race that was paced. This leaves the question, what is different? The answer, to my mind, is in the two types of workouts I have highlighted above. So if you are asking do I think that if the runners of the past were doing more of these sessions they would have been faster my simple answer is yes. I do not believe it is likely or even possible that someone like Craig Virgin could have worked harder than he did and he ran 8:40.9 for two miles in high school so it is also unlikely that any number of athletes are significantly more talented than he was. Yet we have a number of guys running much faster. Some of this could be chalked up to faster races. I have no doubt that at times in his career Craig could have run in the 13:10 range if the race had been going at that pace but I find it hard to believe simple pacing would drop him 20 seconds. So what are the likes of Ben True, 13:02, Matt Tegenkamp, 12:58 and Chris Solinsky, 12:55 doing differently? I believe the answer is fundamental tempos and max lass workouts. Though I' imagine that none of them call them by those names.
Pillar 3- Threshold workouts. Traditionally this would be your half marathon paced tempo run. I would include those but I personally focus much more on alternation style intervals. I believe these are far more effective in improving threshold as well as overall fitness. Your threshold or steady state pace is the single most important fitness maker in deciding your success in races from 3k to marathon. If you have a high threshold you will race well. If you have also do some decent specific workouts with that high threshold you will race very well. Raising your threshold is how you make a pace that is hard to run for longer than a few hundred meters into something you can run easily for a few miles.
Alternation workouts for threshold included things like Aussie Quarters, http://nateruns.blogspot.com/2014/12/workout-wednesday-australian-quarters.html, Moneghetti Fartleks, http://nateruns.blogspot.com/2015/01/monaghetti-fartlek.html, or Renato Canova style alternations, http://nateruns.blogspot.com/2015/02/workout-wednesday-alternations-for-5k.html. Basically any workout where you mix in intervals of faster than half marathon paced running with quick, faster than regular training paced, recoveries.
Threshold workouts are closely related to the fundamental tempos in terms of the changes that they create in your body. Improvements in one of these workouts will lead to improvements in the other. If you find yourself plateaued in either of these work types very often the answer is to focus on the other. For more than a year early in my career I could not find a way to run a threshold run under 5:10 per mile or so. If I did one mile at 5:00 pace it felt painfully easy but somewhere between 3 and 5 miles it became incredibly hard. Then I found fundamental tempo's and within a few months I could do 5 mile tempo runs at 4:50 per mile. In the more then a decade that has past since I have seen this connection in my training again and again.
Pillar 4- Speed work. This is NOT anaerobic work. This is work focused on teaching the body to run fast while remaining relaxed. This can be as simple as strides. It can be done in great volume or in very low volume with great regularity. I try to never let a week go by where I don't do at least some session for speed. Joe Vigil, who is a much better coach then me, tries to never let a day go by where it doesn't get touched on.
Nearly everyone runs fast in their training but too often we fall into the trap of thinking fast and hard are the same thing. Much like the workouts in pillar one speed sessions should not be hard. They are more than an easy day but not nearly a hard day. You can address the need for this type of work in a ton of different ways. You can do sets of strides, 4 to 15, as often as daily or you can do short hill sprints, under 15 seconds. I often do longer sessions like 30x100m or mix 10 to 20 second high speed bursts into a regular run with full recovery between efforts. The two keys to this type of work are a focus on good, relaxed, powerful running form and taking full recovery between efforts. It is easy to slip into not taking enough rest between these sessions, it is all but impossible to take too much recovery.
There you have it. As I stated above I don't expect this to be the only training you do but I do expect that this is what your training schedule should be designed around. If you are regularly working on all of these systems you will do well. If you then start to put them into a well designed progression of increased volume and periodized adjustments you will be truly training and not just running and you will see some impressive results.