Imagine a country path, D, in miles in length, which a person walks

Imagine a country path, D, in miles in length, which a person walks at a steady pace in T hours. Suppose that a straight line represents the cartesian graph of time (T) versus the distance (D) for this walk. What is the slope of this straight line graph, and what does it represent?
Can you think of another example, which, if visually depicted, would have a straight line for its graph?

Minimum words - 200 words (at least). Good grammar and logic of presentation required.