A term used a lot in all areas of athletics is sport-specific training or training specificity. This refers to the fact that you improve most in physical endeavors by training for them doing specific movements which mimic the movements in that activity or that you do the activity itself.
It means that runners need to run in order to get better at running. Cycling, swimming, weight training, yoga, elliptical training, etc. do not make you a better runner. Running makes you a better runner. Furthermore, if you want to run fast, only fast running makes you run faster. Slow training begets slow racing.
Do not jump to the conclusion that cross-training is ill-advised. Cross-training may be beneficial in overall strength or improved balance between muscle groups and thus just perhaps assist with injury avoidance. These may indirectly help you run better if they help you stay healthy e.g. not injured, and so more consistent in your running training. And novice runners will generally benefit far more from cross-training than elite or very experienced runners.
One of the most wonderful examples of training specificity comes from Olympic level competition. In the late 1990s Kenya wanted to field a Winter Olympic team. Philip Boit an elite middle-distance runner trained with elite Norwegian skiers in preparation for the 1998 Olympics. The 10k cross-country winner stopped the clock in 27:24 (Bjorn Daehlie, NOR). Philip came in last in 47:25. Four years later he finished 66th out of 71 starters. Despite a world class VO2max and middle distance running race times the results vividly demonstrate training specificity. At the Olympic level, it requires years of preparation doing sport-specific training. (As an aside, it also vividly demonstrates how even a lofty VO2max is such a poor predictor of performance).
As you can guess, the first and foremost specific training for running is running. To neuro-muscularly train your body to run fast, you must run fast. Remember, all longer distance paces are some fraction of your maximum speed. It’s been shown in research now that as distance runners improve their maximum speed, all other distances will improve along with it. Various drills with skipping, bounding and lunging improve leg strength and can be integral to decreasing foot contact time which improves speed.
The next best thing to running is aqua-running (running in a pool). You optimize this workout by doing interval type efforts. All other cross-training is ok but not running specific. Weight training and exercises should focus on either your core muscle strength or single-legged drills and exercises. Core strength reduces excess motion therefore improving efficiency through less energy wastage. One-legged workouts isolate running specific motions as well as improve symmetrical development of your body. As an observation, be careful not to overdo weight training. Go ask any Kenyan runner how much weight training they do. Got the message?
Bottomline, if you want to be a better runner – do running specific training.