As with any behavioral standard for any group, you will find a range of commitment and compliance with group norms. A low-commitment church member will ignore a dozen other behavioral norms that someone else might care about more than going for a run on Sunday.
But in general, no, I've never been aware of people who are both committed Mormons and dedicated runners sneaking in a run or even a cross-training session. I'm sure it's happened, but I've known enough Mormon runners that I think it's pretty uncommon. I'm also familiar enough with the issues involved, and it just wouldn't make sense to sneak in a run.
For a major race, I have seen runners decide that running is preferable to not running: think Olympic marathon, for example. My own son decided that he could justify running at NXR one year. But there's no sneaking about it; you make a decision and go with it.