Johns Falls Trail to Monument Street is probably the best option here. The Johns Falls Trail can feel a bit random towards the end; Falls road turns into Lanvale, then you hook a right on the sidewalk for St Paul (there are painted markers on the sidewalk but easy to miss) to go behind Penn Station. L on Mt Royal, still on sidewalk that turns into a protected lane as it becomes the Fallsway. When you see health care for the homeless on your R, hook a left on a side road that leads to the beginning of the protected lane on monument. It's a very nice route and easy to follow after you've done it once.