Andy,
Many longer training plans will alter these attributes in alternating periods. So for example... if you had a 52 week plan, you might divide it into 4 week periods. Your "base" super-period might be made up of 3 - 4 week periods, Base1, Base2, and Base3. In Base1, you might use the theory of mostly longer training, at lower intensity. Then Base2 you might switch it up and do shorter training with higher intensity. Finally in Base 3 you might do mid length, mid intensity.
If you want to see improvement, ultimately you have to build up and increase your effort. Improvement comes from over-extending your ability, then giving your body a chance to recover. That over-extension might be in time duration (going longer than you've ever gone before) or it might be more effort (going faster, pushing harder, doing more hills, etc).
Depending on what your goals are, you will choice different systems and techniques to create this over-extension.... If you want to participate in longer distance races, then clearly you need to focus on increasing your distance/time, and effort over distance/time. If you goal is to be faster and faster at shorter races, then you'd want to focus less on time and more on effort.