It's not hard science, but I've only run faster since I stopped doing LSD, at all distances, while gaining lean mass and a lot of strength and power. My mile the season I did LSD was 5:35, the season I didn't do LSD it was 5:12, and I was 20 lbs. heavier with a 50 lb. higher power clean, and was training for swimming and the shot put at the same time. Look at Dean Karnazes, he's considered muscular by distance runners, and weighs only 154 lbs. I consider myself fairly thin and I'm 173 lbs.
When I want to go slow and long, I go for a long walk. You get the same mental benefits of relaxation and spending time outside, without the joint trauma and loss of strength and power.