Our experience as coaches tells us that the most gain in training is by doing as much specific training as possible. BUT this is a high risk option. Maybe, if you make your living road racing. For the rest of us mortals, perhaps there are some other options. How about considering some things that will not necessarily make you a faster runner, but will improve your overall fitness, health and enjoyment, reduce your running injuries and help you to be a lifelong runner.
Most runners think that they are highly fit.
In fact, the elements of fitness are cardiovascular endurance, muscular endurance, muscular strength and flexibility. Runners excel in the first two, often to the detriment of the last two. This fact, coupled with the very specific muscle development of the lower body contribute to many running related injuries. Psychologically, many runners are highly focused and dependent on running. When they become injured, they have druglike withdrawal symptoms and lose a sense of purpose.