Are questions about trail running good for this site? It certainly has relevance to the outdoors, but is it appropriate here? If not which SE site is the right fit? I checked out the Physical Fitness SE and the Sports SE to see if it might be over there. And I did find some information, but really not enough to be conclusive.
Trail running is a multidisciplinary activity, so whether the question itself is actually on topic here depends on the context.
Physical Fitness SE
If the question is about physical fitness or training, it is a much better fit on our Physical Fitness site.
The Great Outdoors
But if the question is primarily about elements of hiking or trails, or concerns in dealing with mountainous terrain, or perhaps the techniques of ultralight backpacking, then this is the site for them.
So the entire subject of "trail running" isn't entirely on or off topic for this site. It depends on the context of what they are asking.
In my opinion, yes, it certainly belongs to The Great Outdoors. Most of questions related to trail running are likely quite specific to the aspects that make trail running unique; namely it being outdoors and in terrain. Therefore, trail running is on-topic for TGO.