There are a number of factors that influence your running speed. Assuming you are already training on a regular basis, here are 4 reasons why runners struggle to get faster. These are all factors we can control.
Body Composition – If you are carrying extra weight in the form of bulky muscle or fat, this will definitely slow you down, especially if the added weight comes in the form of fat. This slow down is exponential as the distance increases. The solution is to take weight off gradually through proper nutrition and balanced diet. The off season is the best time to focus on active weight loss.
Muscle Imbalances – When placed under load, your muscles attempt to work together in unison to move you forward as efficiently as possible. If one or more muscle groups is not cooperating, the entire kinetic train is derailed and other muscles will compensate. Your efficient movement will become labored, eventually slowing you down. The greater the imbalance, the greater the degree of slowdown. I would suggest a Functional Strength Assessment to determine your imbalances if you don’t already know them.
Too Much Speed – The problem with too much speed within your daily workouts is not only does it increase your chance of injury, but it also limits your ability to run fast. You end up running “fast” but not fast enough to bring about the changes you are looking for. Keep you speed very focused and purposeful. Not all of your training runs need to be personal bests. Limit your speed work to quality sessions that are very focused and fast. I would also suggest working on running form drills.
Running at the Same Pace – While running too hard on a regular basis will limit you, so will running every run at the same easy pace. Varying your speeds will keep you form being lulled into a comfortably slow pace. Slow runs have their place but if that’s all you do, that’s a perfect recipe for slow racing. If you are new to speed work, then I would mix in short bursts of speed within the context of your regular runs. For example, an acceleration (not a sprint) of 60 seconds every five or six minutes within your regular runs will help prime the body for more structured speed work in the future.