This happened some months ago. I went lucid due to some thing I was reacting to in the dream, and I knew that I could get away from whatever it was by running. Bursting into a sprint while focused so hard on accelerating, my legs actually moved in real life. I woke up mostly because of it and lost the dream. The mental effort I exerted in my dream forced my physical legs to move because it was so great. It overcame the natural mode of sleep paralysis that my body was in. Awesome! Well that is what I'm going to go with since that seems to be the simplest explanation.
In this lucid dream, I was able to run super fast. It started in a park where I was walking along with some other people. There was a stream coming off a hill, as from a spring. It was nice there as it flowed down the gently sloping rock face & meandered along next to the sidewalk a ways. The dream was becoming more vivid for me and I felt like going for a run. I started off at a brisk jog, but then soon sped up to what would be an all out sprint in real life, but going in excess of 20 mph. I was able to keep up this mind numbing pace for several miles, maybe 3 to 5 miles, without even getting winded. The normal signs of running were not there, my heart didn't beat any faster, nor did I need to breathe any harder. It was like I could run forever and not tire, but I did stop eventually as the thrill of it waned. As I ran I passed several people on the walkway, even people on bicycles. No one shall catch up to me, even if anyone was even trying.