I'm guessing that no one who reads this is a sprinter, so this will be short.
Running at maximal speed uses muscle fibers not often used, but which can aid at slower speeds. Most people can accelerate to top speed in 5 seconds (world class sprinters can take 15) and this is as long as a sprint need be. Three or four sprints is enough for non-sprinters to get benefits.
Running sprints up a steep hill has become popular and it has two positives: it requires one to lift the upper leg closer to horizontal than most runners ever do, increasing range of motion and it also decreases impact forces, which lowers the rate of injury. Hills should be under 12% grade or one's form usually alters.
Running at top speed on a slight downhill is the only form of assisted sprinting most people should try. Gravity can cause one to run faster than what their maximum speed would otherwise be and this can lead to gains in speed on flat ground. One must be very careful when doing these to avoid injury.
So, once per week, run a few all-out sprints of 40-50 meters. The time spent in recovery is unimportant.
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