Kampinos National Park is an extensive park, covering 385 square km., in the north-west outskirts of Warsaw. Best accessed by car or a 1 hour bus ride, this is a good spot to escape for some nature and trail running. The trails are mainly wooded trails, but also some swamps and meadows. There are several monuments and commemorations of important (and tragic) historical events that occurred in Warsaw and around this park.
There are extensive trails for walking, nordic skiing, and the 200 km Kampinos Cycling Trail. For running, there are many options. Warsaw Running Tours recommends the Yellow trail from Truskaw – Leszno featuring 19 km of diversified landscape inside the forest. The Blue trail in Stara Dąbrowa is ideal for hill running. See the map link for more options.