Training in phases was most notably put forth by Arthur Lydiard in the the late 1950's and early 1960s. Now there were other coaches before who were already doing this but he was most responsible for popularizing this and for creating the idea of doing very different training depending on how far from your race you are. Lydiard advocated a base phase for building the aerobic system, a hill phase for building the mechanical strength, an anaerobic phase for building the anaerobic threshold and prepare for the specific demands of racing and finally a race phase to continue specific workouts and start doing races to sharpen up for and specifically prepare for your goal race or races.
On the other end of the spectrum would be athletes from the Pat Clohessy school. This includes athletes he directly coached, like Rob de Castella, but also athletes from systems through out Australia who were heavily influenced by Clohessy and his success. These programs call for almost the exact same training each week of the training year. So if you look at a schedule from one of these athletes from 5 months out from a race it looks just like it does 5 weeks before their goal race. The thing is they do in fact adjust their training. Which workout is the focus and which workouts are treated as lighter efforts. They adjust how they perform certain workouts to target specific fitness goals. So it is that in modern training in one way or another are using phases of training to produce their best results.
Most modern programs are closer to Clohessy than to Lydiard. Which is to say that they heavily use all types of training in all phases. It is a fallacy that Lydiard did only one type of training in each phase but his phases were much more focused on individual types of work. The reasons this type of starkly different phases has largely faded out of use are varied but the big three reasons are that somewhat greater gains can be made by doing a more mixed phases, that if you have a set back along the way it is easier to get back into things without a major time set back with more mixed phases and finally Lydiards athletes were very slow in their base phase. In modern running and racing athletes are expected to be world class every time they lace up their racing shoes. It was not uncommon to see a Lydiard coached athlete who was at his best a mid 13:30 5k runner race a 5k in the mid 14's while perfectly healthy in their base phase. If Galen Rupp regularly ran 14:00 5k's at an indoor meets in January he would stop getting his normal fees and he would have to deal with all sorts of performance cutbacks on his shoe contract. It just isn't a feasible system for many top pros. Similarly with three competitive seasons it is very hard for a college or high school coach to have athletes under perform in one season, completely skip another to produce great results in there key season. It is not to say if this was the only way to achieve success there wouldn't be coaches doing it but instead coaches found other ways and in doing so found ways of producing equal or eventually superior results through other methods.
For me I generally use three phases Fundamental, special and specific. I sometimes skip the fundamental phase if I am very fit and have come off a strong last cycle with only a short break between. How many phases you use depends on which system you are using. I would encourage you to pick a good program, there are plenty out there, and follow it through once in full fidelity. That means follow the program as laid out and don't make any adjustments. See how it works. See how you react to it. Only then should you repeat it and make adjustments. We are not all Bob Ross who could make a mistake midway and naturally and easily see how to turn it into a perfect part of the final masterpiece. From my perspective better to start with Paint by numbers and branch out once you have a sense of how the whole thing comes out. I'll do a blog on great programs I suggest sometime soon but really there are tons of good ones out there.
So as I said I use three phases and I'll get into them in a moment but I want to highlight that regardless of if you use 6 phases or 4 or 3 or if you do basically the same thing year round and tweak it to match the period of your training the KEY is how you progress workouts. This is tied into having an understanding of what the goal and effect of every type of training and workout that you do is. I'm not saying you need a grad degree in exercise physiology to coach but you should never do a workout because it is a hard effort or because it looks like a good workout. Specifically I'm talking about if your goal is good performance. If you are running and training for enjoyment then I have no issue what so ever with you doing such and such a workout you read about recently because it looks like it would be really neat to do.
First the fundamental phase. This is most similar to a lydaird base. The focus of this phase for me is two fold. First to build basic aerobic systems. This means long easy tempo runs, basic easy mileage and perhaps a few threshold type tempo runs or tempo intervals. The second focus is building the muscular systems. This means strides, short hill repeats, easy circuit training and running over hilly or muddy or sandy or deep soft grass type terrians. This could also include things like running with a weight vest or doing walking lunges, with or without weight vest and perhaps long tempo intervals for volume. An example would be 6xmile at about half marathon or marathon pace with long full recovery. The idea is the reps should be easy aerobically but you get some muscular work. Another example might be doing 10 to 20 k of short reps, 200 to 400m at goal 5k to 10k pace but with full rest so that each rep is very easy to run but your muscles will get very tired. A session at full volume as I just described is drifting into special phase type work but certainly the early efforts where your volume of the workout is more in the 5k to 10k range.
So for me this earliest phase would start very light and quickly build up to fairly high mileage. My key focuses would be on trying to do some sort of muscular work every day. Be this just a set of strides after a regular run or something more focused like some short hill sprints, or hill bounding and springing. My main focus workout would be long fundamental tempos. Here are two links to blogs I have done on fundamental tempos for 5000m and marathon training. http://nateruns.blogspot.com/2015/01/fundamental-tempo-runs-for-5000-meters.html
These fundamental tempos help build general aerobic power and endurance as well as good muscular endurance.
Secondary workouts would including an easy long run. Some sort of hill work, either hill circuits of bounding, springing and other exercises or long moderate effort repeats or long moderate effort continuous uphill runs, if you are lucky enough to have a 3 to 10 mile continuous uphill in your vicinity to use. You can use a treadmill for this type of work but since you don't have to lift your center of gravity the muscular component is largely lost. So for example I did 7.6 miles at a 12% grade on a treadmill in under 1 hour but racing up Mount Washington, 7.6 miles at an average of 11% I was only able to run 1:06 or so.
Additionally you can do your specific muscular type work. For a 10k or longer runner this would be high volume of short repeats at or around goal race paces with full recoveries to avoid it becoming anaerobic. For the 10k or under runner this would be very short 100m to 200m efforts run at or much faster than race pace again with full recovery to avoid becoming anaerobic and done for great volume as much as 5k to 10k worth of work. Sessions may be something like 30 to 50x100m at 400m to 800m race pace or a long session of diagonals or the like. Again it is key that all this work should be done with so much rest that you avoid the breathing getting hard or the feeling of acid building. You want to fatigue the muscles and only the muscles.
Onto the special phase. This is the period where you are trying to build your greatest specific fitness. The main session becomes threshold workouts. You will have done some of these in the base phase but now they should be your main focus. You should be doing at least one threshold run per week. These may be straight up threshold tempos of 4 to 8 miles or bounce threshold type workouts like Aussie quarters, monaghetti fartleks, or alternations or threshold progression runs. Some links to blogs on these types sessions are here…
During this phase you should continue the longer fundamental type tempo running. For the marathon athlete you should do some very long sessions at the same pace that you were running in the base and some sessions of the same 15 to 25 mile range but you should be increasing the pace closer to marathon pace. For athletes focused on shorter races particularly 10k or below you should keep these sessions going but you probably shouldn't really be improving them at all. They should become secondary sessions just there to maintain aerobic endurance.
For all events you should extend and build on your specific muscular work that you started in the fundamental phase. The length of the reps should increase a bit and volume can continue to come up. You can be doing as much as double your race distance worth of work at your goal pace for 5k and 10k runners and as much a full race distance for half marathoners. Marathoners are probably only going to be able to do half to 2/3rds of race distance. The rests should still be fairly long but if you get a bit anaerobic at time it is ok but your main focus should still be on being as smooth and relaxed at pace. For 5k and down racers you should be continuing the faster than race pace type repeats with full recovery and high volume. As with the other distances the lengths of your repeats can increase.
Now for these 5k and down racers I would continue the 100m reps weekly at fast speed doing 3k or more of volume. I would in fact do this in all phases as it does amazing things for relaxation at race pace that enable you to really produce greatly improved race results in these middle distance events.
During this phase you should be doing some under and in the case of 800m to 10k runners over distance racing. Don't expect incredible results in these races but they are a great chance to build racing toughness and skills as well as measure your fitness and get in some good hard workouts to attack your weak points. Ie if you are the type of runner who does better in longer races you should do more under distance racing for runners who excel in the shorter events and struggle in longer stuff you should be doing more over distance racing in this phase.
Finally you enter the specific phase. Here you may do some workouts from the other phases to maintain your general fitness level but these sessions are disincly secondary. At this point all your main workouts should be focused on what I like to call 'building your race.'
This means that you should be doing workouts that are either at or extremely close to your expected race pace. For those training for championship style racing will need to use more paces and variety because they will likely not be running an even pace in their most important races. The biggest mistake I see people make as they approach their big races is that they keep working out faster and faster and instead of building a great performance they start pushing their fitness in a direction that isn't best for their goal. What I mean is if you have an athlete who has run 16:00 for 5k at the end of their special phase who would like to run in the low 15's for 5k. Let say they have been focused on 72 seconds for their goal pace and repeat pace. Which is fine. Perhaps a bit aggressive but certainly close enough to be effective. So perhaps at the end of the special phase they are running 12x400 with 200 jog rest in 72 seconds. The tendency is to keep doing this workout but getting faster each time out, 71's and then 70 and before you know it they are running 65 or even better per 400 on the same rest. They run better but not much and when you consider how much they have improved their workout that is very disappointing. Instead you need to look at your workouts and your goal race and say what is different between the two. So in this example case the difference between 12x400 in 72 with 200 jogs and 5k in 15:00 is the 200 jogs. So each specific workout should be run in a way to eliminate that distance. You can do this directly by increasing the volume of the repeats at the same pace while not increasing the distance of the rests much if at all. So you are far better building towards running 3xmile at 4:48 with the same 200 jog. You should also mix in some other specific sessions that work in different ways. So perhaps a portuguese surge, http://nateruns.blogspot.com/2015/01/tempo-tuesday-portuguese-surge.html , where you run the last 1k to 2k at or just a shade quicker than your 5k goal pace.
So there you have how I break down my cycles and how I suggest you should always be building your sessions over the course of a full training cycle to produce your best performances when they matter and reach your full potential over time.