I do trail running too. I wouldnīt add any unnecessary load on my body (if you donīt need the pack for hydration etc.). Itīs bad for the joints. When I run short trail and want to increase the effectiveness of the training I increase the speed (or sprint some sections of the trail). Sometimes I compose series of pushups (without any breaks) into the run.
Or I set up a countdown and try to reach the destination in time. Itīs pretty motivational.

If you really want to wear the pack though I would look around for some specialized hydration packs for running. I will search in my bookmarks and post something - I did search on this because I wanted the pack for longer runs in order to carry more water.