Our bodies respond to training stress by being temporarily weakened, then growing back stronger, a process called supercompensation . There are different ways of training that produce different training stress and result in different improvements. Training modifies different systems in the body, including cardiovascular, muscular, biomechanical, neurological and hormonal. This article gives a high level overview of the different training types; future articles will look at some of these in more detail.
Easy runs/recovery runs/junk miles
Many runners do the bulk of their miles in 'easy' runs. These runs are 'easy' in the sense that they are not so fast or so long that they push the limits of the runners ability. While there can be some benefit to this type of run, some coaches consider these runs as 'junk miles'. The runs are not fast enough to significantly improve aerobic capacity, maximal speed or endurance. There is little evidence to support the idea that an easy run on a rest day improves recovery [2, 3]. Adding an extra 20% distance at an easy pace (from 4 to 6 days/week) did not improve marathon times in a study [5,6].
Interval training is alternating high and low intensity exercise. The high intensity might be a slow jog for a beginner or a fast run for the more advanced. The low intensity might be a slower run, a walk or laying on the ground. I believe that interval training is the most important type of workout, for everyone from beginners to elite athletes. For beginners, it seems that there is a perception that interval training is in some way taking the easy way out, or that walking in a run means you are not a 'real runner'. Nothing could be further from the truth; real runners do interval training! Interval training is the most efficient ways of improving aerobic capacity and maximal speed. Interval training is also great in hot weather - the low intensity period allows for cooling off.
Everyone has a limit on how far they can run. The long run is the key workout for increasing distance and is part of any reasonable marathon training plan. The distance that can be run is dramatically increased by taking walking breaks; you can go much further than with continuous running. Generally, a long run is at an easy pace, often well below the target race pace. However, a number of running plans include some type of higher paced running. I would advise most runners to have one run a week longer than the others. The length of the long run, how quickly to build it up, etc, will be the subject of another blog entry.
There is a lot of variety to hill training. It can be used with interval training, running hard up the hill and easy down to build fitness with lower impact. Running hard down the hill and easy back up again provides adaptation to prevent downhill muscle damage (more on that in a dedicated blog entry). Also a long run can be hilly to prepare for a long, hilly race.
A tempo run is generally a run for 20 minutes at a pace that could be maintained for a maximum of an hour (83-88% VO2max)  . Jack Daniels claims tempo runs 'are one of the most productive types of training'. However, I have not found tempo runs to improve my fitness in the same way that other types of workout do, nor have I found any scientific evidence for the benefit of tempo runs over intervals. In fact, the evidence I have seen indicates that although tempo runs are better than no speed work, your time and effort are better spent on intervals.
Racing can be used as a form of training, and for some people this is their preferred way of improving their speed. I think that a longer race (marathon or longer) does too much damage to produce a fitness benefit.
 Neuromuscular fatigue and recovery in elite female soccer: effects of active recovery.
 Rest v Active Recovery
Jack Daniel's Running Formula, 2nd Edition, pp 111
Preparing For Your First Marathon
Long slow distance training in novice marathoners