If you're trying to figure out how to balance out lifting and running, I am still working on it. What I can say is: you should do both. Cross-training is really important for becoming a strong, well-rounded athlete and also helps you to stay injury-free. If you consider what you're doing on the trails, you'll realize just how many muscles you're actually working. The act of trail running demands different muscle usage along with uneven, varied terrain with a lot of ups and downs, which means you're using different sources of power all the time.