Okay, ran into a major bug (I think) of sorts... The "dogs" in the Overgrown Tunnel are moving significantly faster than the other moving entities in-game, including Mara... After several attempts, I have yet to get to the key in the back of the tunnel. The furthest I've managed to get is the first turn towards the "north".
Quick edit to verify: Yes, I have tested with sprinting. They still have enough zoomies to out-pace. Maybe they need to switch to decaf or something...