Forgetting for a moment that you're Lassie, you turn tail (literally and figuratively) and run away from the burning building. Dogs, mainly due to both smaller size and much larger quantities of body hair, are entirely more flammable than humans. Being highly aware of this, you run and run until your little furry legs can take you no further.
Panting, you take a drink of cool, refreshing water from a nearby stream. Based on the position of the sun and mountains, and the time of day, you appear to be nearly 25 miles away from your home. For a brief moment, you wonder at your innate knowledge of geography and distances. Then you remember that you are Lassie, and it's all self-explanatory.
From a short distance away, you hear the voice if little Timmy calling for help. You can't help but wonder at finding him this far away from home, especially after you ran 25 miles in a random direction. You peer in the direction of the call for help, and notice a brick-lined well, off in the distance. The bucket and rope are nowhere to be seen.
Just like him to be trapped in a well. Again.
That little ingrate.