Muscle memory can really help to improve your speed.  One technique that works really well for me is running downhill.  If you are fortunate enough to have access to a treadmill with a decline option, it’s much easier than finding a hill on your everyday running course.   The benefit of running downhill is you can build muscle memory at your goal pace by using less energy and allowing you to run longer.  As an example, running an 8:00 mile on flat land or with hills may have your heart rate around 155-160 BPM. By running downhill, your heart rate will be lower to maybe 130-140 BPM.  You are now less likely to experience overtraining or fatigue and you’ll train your legs to turnover at the rate of your goal pace.  Try it!!

Read the below article by Alex Hutchinson from Runner’s World which describes his experience with muscle memory.

It took me more than five years of hard training to break 4:00 for 1500 meters for the first time. Not long after, I missed almost two years of training with a knee injury. When I was able to start running again, I wondered (and worried) whether it would take me another five years to regain my form. In fact, I was able to break 4:00 again after only a few months of track workouts and ended up running 3:48 that summer. How could something that was so hard the first time be so easy (relatively) the next time

Source: Muscle Memory and Endurance | Runner’s World