There are over 3 activities in the Trail running category.
The runners avoid the city and run off-road. This is easy on the joints and the trail run in the countryside provides a mental motivation boost. Almost incidentally, it helps to increase fitness and performance.
There are rather easy trail running routes at Bettmeralp, Flims, Suls and in the Jura (Gorges de l'Areuse) Moderate routes can be found near Alpstein, Glattalp, Grindelwald, Klöntalersee, Le Moléson, Maderanertal, Pizol and Ritom. More challenging routes can be found on the Brienzergrat, in Fionnay, Lenk-Iffigenalp, Lobhörner, Melchsee-Frutt, Val Ferret and in Zermatt.
The trails off the asphalt roads give way more and are easier on the joints. Hips, knees, ankles and even shins do not have to be exposed to the shocks of an asphalt surface. Trail runners must adapt quickly to the changing terrain. This improves balance and stability.
The feel for the ground becomes more pronounced over time. Trail running not only supports core stability, it also provides a higher degree of mobility over time, addresses a great many muscle groups, and is even considered a hidden strength workout. The fresh air without exhaust fumes is also good for the lungs and cardiovascular system.
When trail running at night, you need a very good headlamp. It should have at least 300 lumens, because it is darker off the road than we are used to. In the dark you should never run alone. It makes the run safer and it's more fun with two people anyway. Make yourself visible by wearing bright clothing or clothing with reflective stripes. Maybe there are others on the way. You should also know the route you are taking at night. Safety always comes first.