I was actually a slow walker until last July when the first fellow aspie I ever met in real life showed me how to walk properly. I was hanging out with her all day, and I asked her to deconstruct me as she went. At one point she broke down the way that I walked. Apparently my hips weren't moving at all. You're supposed to put your butt and hips into it. You're also supposed to stand up straight with your head up.
Once I learned how to walk properly, at the age of 28, I became a fast walker.
I also walk long distances routinely. This is because I've never had a car, grew up in Los Angeles (nobody walks in L.A. ... except me!), and currently live in Las Vegas (which is built like L.A., though not nearly as big). In particular, I grew up in the northern mountainous area of L.A. County, and frequently went on long hikes, off trail, over extremely rugged terrain. I probably walk faster over loose rocks and loose dirt than I do on a flat sidewalk.